any more than the others, not 'much more'.

Again, that might be true in a textbook-economics world, but in reality people have other criteria besides salary when choosing a job, and don't always know about all the available options, etc. A company that pays blond-haired people slightly more will have an advantage when hiring blond people, but I wouldn't expect it to be the sort of advantage that means they simply have the pick of the best.

yes, that's how it works

So on what grounds do you say that paying blond employees the same as everyone else, rather than 20% less like everyone else, will "reduce the rate at which this inequity is wiped out"?

No, the analogy was otherwise.

Oh, OK, so your proposed mechanism is that paying more means hiring fewer people. I agree that this will happen to some extent. But:

If our hypothetical employer keeps its blond-employee-hiring budget constant and hires fewer blond-haired people precisely in proportion to the bigger salary it pays them, then the total flow of money from that employer to the blond-haired population is independent of the salary. In that case, to a reasonable first approximation, they will be as much use to blond-haired people whether they pay the market rate or (let's call it) the equitable rate.

But of course they won't do that, for two reasons. First, the budget that might be fixed is their total hiring budget, not their blond-hiring budget. So some of the people they don't hire will be non-blond people, and there will be a net benefit to blond people. (You can think of this, if for some reason you want to, as a reduction just in hiring blond people, plus a transfer from non-blond to blond. The latter is probably an overall utility improvement since blond people have been doing worse as a result of prejudice.)

Second, I do not believe that the fixed-hiring-budget model is accurate. If employees become more expensive, then as well as hiring fewer employees employers will spend less on other things and maybe take less profit. So, if again we start from the fixed-blond-hiring-budget baseline, this amounts to a transfer from the employer to the blond population (which they are choosing to make on moral grounds; this seems to be clearly their right).

No, you probably couldn't

It was only a throwaway example but OK, I'll be more detailed: be brusque and rude to people outside work and to underlings at work, be polite but very focused with peers at work, and kiss the boss's ass.

you're giving them a better job than they can find otherwise, that's quite decent.

I don't think this is obvious. Let's consider an extreme example (warning: mindkilling potential lies ahead, proceed with care). Suppose you live in (a cartoonishly-simplified version of) Nazi Germany where default behaviour to Jews is to beat them up, summon the Gestapo, and have them shipped off to concentration camps. If you see a Jewish person and beat them up a little less than most people would before summoning the Gestapo to have them sent off to Auschwitz, is that decent? I say: of course not. (Even though otherwise they'd soon have been found by someone else, beaten up worse, and sent to Auschwitz, a slightly worse outcome than you gave them.)

From which it seems to follow that treating someone in a particular way isn't guaranteed to be decent merely because it improves their outcome a bit compared with what they'd have got without you, and compared with what they'd have got if you'd behaved like everyone else does. If what you do is bad enough, it can fail to be decent despite passing that test.

Now, obviously offering someone a job that pays them less than they'd have been paid with different-coloured hair isn't in the same league as beating them up and sending them to Auschwitz; the above example is intended merely to make the logical point in the foregoing paragraph as vividly as possible. I don't think it's obvious what the threshold is for decent behaviour in the situation we're actually discussing (either the imaginary one with blond hair or the real-world one with double X chromosomes). I think there's a reasonable case to be made that paying some subset of the population less simply because they belong to a discriminated-against group and you can therefore get away with it counts as indecent.

[fair-trade] t-shirts will cost more / be produced more inefficiently ... / be inferior

I agree with your general argument, but that particular example isn't true IME -- ISTM fair-trade products are cheaper than non-fair-trade products of the same quality (e.g. the fair-trade t-shirt I own cost me €3 IIRC, and after three years I've owned it it doesn't show that much wear and tear). I suspect customers refrain from buying them out of ideological reasons or something.