I wrote my honors thesis about a different aspect of this same issue. There are basically two ways you can go here. One would be to radically circumscribe the egalitarianism and not have the state provide for things like mental health services (Will's example) or public education (including vouchers, my example). The other way is to try and alter the Rawlsian conception of a “reasonable comprehensive doctrine” to incorporate some epistemic norms along with some ethical ones.
The problem for Matt is that the latter option abuses what Rawls means by “reasonable” and, in any case, pretty much destroys the appeal of his policitial liberalism.
Second point first. . . The mainstream libertarian rap against late Rawls is that he packs so much into the idea of a “reasonable conception” that he saps his liberalism of genuine respect for the fact of pluralism. “Reasonable” here is defined so restrictively that other forms of liberalism, like libertarianism, are excluded by fiat. This isn't a bad point, but it's hardly enough to dismiss Rawls. The right thing for a libertarian to do, I think, is to just demand that the notion of a “reasonable conception” be broadened in certain ways, and that once it is so broadened, to argue that libertarianism satisfies the desiderata of political liberalism better than egalitarian or welfare-state liberalism.
Now, to the idea of incorporating certain epistemic norms into the idea of the “reasonable” . . . Clearly, this is going to run a Rawlsian head-on into the argument, mentioned above, that he's just faking it and doesn't honestly care about a distinctively political liberalism that can really handle the fact of pluralism. It's pretty clear that one can't just make ad hoc additions to the notion of the reasonable until one gets one's fovored conclusion.
More importantly, however, is that “reasonable” in Rawls is a practical, not an epistemic, notion. A reasonable person is one who has an effective sense of justice, and is therefore disposed to propose, accept, and abide by fair terms of cooperation, and to govern the rational pursuit of their ends by these terms. Reasonable people are able to achieve cooperative outcomes that are unavailable to to purely maximizing (unreasonable) rational folk. The overall stability of a well-ordered society requires adherence to reasonable terms of association. Now, as I understand it, a reasonable comprehensive conception is one that is consistent with reasonable terms of association, that is, terms of association that people recognize to be fair, in their ineterests, and therefore worthy of adherence.
A reasonable conception, in this sense, could be one that is chock full of crazy beliefs. Indeed, a main point of political liberalism is to accomodate people of conflicting metaphysical views, including those derived by faith, authority, or whatever irrational means you like. Even if people have absolutely crazy views about mental health, such as the view that craziness is caused by demonic posession, they still have a reasonable comprehensive conception, in the relevant sense, if this conception is compatible with respect for and compliance with reasonable terms of social cooperation.
So I think we're left with the first of Matt's options. You've got to “radically cirscumscribe the egalitarianism” and keep the state out of mental health (to use my example), or else choose one: (a) accept state-funding of homosexuality therapy or (b) abandon political liberalism.