Eligius is already committed to the elegant kludge of Address as username
You see that as a kludge. I see it as an elegant solution.
Since the out-of-band data pathway has already been exploited, why stop there? We have one obvious piece of data that could be passed as well by using the same trick.
The issue here is, someone else can screw up your payout value by mining witha different setting with your address... which one should be used?
I don't really see a problem here if the value is sensibly constrained, perhaps more than my original suggestion, say 2 digits from 0.1BTC to 9.9BTC. Use the last value that came with a share (so the trickster must actually pay you in shares for the privilege of changing your threshold), I would be interested in a scenario in which such a prank would matter to the recipient.