As I mentioned yesterday, when one refers to liberals in the context of ethics it means something quite different than it does in reference to modern United States politics. Liberal ethical systems are based on the idea that people have certain rights and the moral thing to do is respect those rights. Historically, the idea of liberal ethics can be traced back as far as the Magna Carta which codified protections that some people had from the power of the monarchy. By its nature liberalism is closely tied to the concept of "rule of law," which refers to the notion that the highest authority in government should be a stable, public, codified set of laws rather than the dictates of a leader or governing body.
On the other hand, I don't know of any liberal political philosophy that predates Thomas Hobbes. If you have ever heard the phrase, "nasty, brutish, and short," used to describe human lives you are at least minimally familiar with his philosophy. He is also the inspiration for the name of Hobbes in Calvin and Hobbes, just one of the many reasons that it is my favorite comic! Anyway, Hobbes argued that left to our own devices humans live pretty miserable lives, see above "nasty, brutish, and short," and in order to avoid this barbarous conditions we cede our sovereignty to some central government which, in turn coordinates the actions of its citizens to minimize casualties. If you are familiar with the saying, "those who sacrifice freedom for security deserve neither," you have about the antithesis of Hobbes' philosophy. Of course, the freedoms that Hobbes asks us to sacrifice are those like our sovereign right to kill other people if we deem it expedient, and the security we gain mainly comes from the agreement of our fellows to sacrifice the same right.
More recently, and more recognizable in the United States philosophy of government, John Locke was another political philosopher. In contrast to Hobbes, Locke tends to focus on those individual rights in which the government ought not interfere for citizens to lead fulfilling lives, rather that which individual rights ought be forgone in order to lead fulfilling lives. This difference is quite understandable given their historical contingencies, Hobbes lived in an era of great turmoil and violence in England and, rather than trying to justify some existing despotic power, he was simply attempting to discern what form of government could provide safety to its citizens. On the other hand, Locke was a rebel sympathizer, and was attempting to provide philosophical justification for overturning the government.
One thing the two authors have in common is that rights take precedence over government. In Hobbes government is an essential outgrowth of the most fundamental right that each of us possesses, the right to protect our lives. On our own we cannot be assured that we will prevail against whatever threats, be they human or natural, assail our very livelihood. Grouping together in governments is the only logical course Hobbes sees to secure a modicum of freedom from all out warfare against all our neighbors. Locke, by contrast, sees the government as a threat to our rights, a perspective that also requires that rights be more important than the current government.
Of course, Locke sees rights as including much more ephemeral concepts than Hobbes, whereas Hobbes rights include things such as the right to do whatever is possible to protect one's life, which by its very nature cannot be taken away, as there is no way to prevent someone from exercising it, Locke includes things such as property ownership in his notion of rights. Whether or not owning property is a fundamental right, it is quite possible to prevent someone from exercising that right. I have always felt the phrase, "we hold these rights to be self-evident," to mean that the founding fathers couldn't really come up with good reasons to justify the rights that they were about to list. Taking liberty for example, even if I do have a right to liberty it is quite possible for liberty to be taken from me, so in what sense that right is "self-evident," is, in fact, not evident to me.
To bring it back to the topic at hand, mandatory organ "donation," it seems quite obvious that an ethical system that places personal welfare, as represented by rights, above communal welfare, represented by government, could effectively argue against taking someone's organs, even if would benefit many other people. Of course, liberal systems do not take this to its logical extreme, they are not libertarians after all, and permit individual rights to be infringed upon as long as it is a result of some infraction on the part of the person whose rights are to be curtailed. As such, it would not be incompatible with such a system to have mandatory organ "donation" from people convicted of some form of crime. Nor would it be incompatible for such a thing to be strictly forbidden to do to anyone of course, it would just depend on the specific code of rights.
The question then becomes how do we explain the divergence between the Trolley Problem and that of mandatory "donation" in a liberal system. I doubt anyone would say that by pulling the switch to divert the train away from five workers, to the detriment of one worker, in some way violates the workers rights. And, if it does, wouldn't leaving the switch as it is violate the rights of the five workers who died? If there is no way to satisfy all the rights involved in a situation, then the idea of a right based morality loses some plausibility, unless one ranks different rights in some order of priority. In an attempt to explain one possible difference between running a train into someone to save five other people and taking the organs from someone to save five other people I may talk a bit more about my man Kant tomorrow. However, are people bored with this rather lengthy ethical consideration of trains and organs? Would you rather I talked about something else, and, if so, any suggestions as to what that might be?