In The Myth of Ownership, Murphy and Nagel recognise that one can indeed put intrinsic positive moral value on markets. They suggest one might the market as "a mechanism that makes each of us as economic actors responsible for the allocation of effort and resources in our own lives, and that makes the benefits we derive from those choices systematically dependent on their costs and benefits to others". (Murphy & Nagel, 2002, p.68). Despite this, they conclude that markets cannot be inherently justified, at least as the sole locus of the distribution of income in society, because people have unequal starting points in terms of their upbringings and abilities. This, they suggest, means that some amount of redistribution is also needed in order to correct for the initial iniquity.
I wonder if one might take a different tack: one focused on virtue ethics. The idea is something like this: one thing we might want out of our situation in life is the capacity to develop specific virtues. Moreover, the virtues we wish to develop will vary from person to person. Courage is an important virtue in some contexts, but for the average citizen in a liberal democracy it is going to be of little to no use. Integrity is one of the most important virtues of a statesman, but has little relevance to most people's everyday life. Generosity becomes a more important virtue as one obtains more to give away - if one is poor enough then it generosity may in fact be a self-destructive vice.
In particular, there are a number of virtues which are most relevant to those who suffer or have suffered. Being forgiving is perhaps the most obvious of these.
With this, then, we may be in a position to argue that havng a disadvantaged upbringing allows one to cultivate certain virtues which would not be open to oneself otherwise.
There are a lot of problems with this view as it stands. First, there's the sheer unpleasantness of the suggestion that people should suffer in order to "be better people" afterwards. Second, presumably by having the disadvantaged upbringing one loses the ability to develop other virtues which are most available to those with advantaged upbringings.
Perhaps this argument can be given legs, perhaps it can't. I think most likely the latter, though I'd be interested to see attempts at developing it.
A related - and in my view much stronger - argument is inspired by an article in Ethics last year arguing that the "shape" of a life has moral value. In short, it is better to start off badly and end up well than to start off well and go downhill. Combine this with the empirical fact that, for a variety of reasons, people tend to get richer as they get older, and you could have a very solid defence of economic inequality.