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.
I submit that it is both. The "right" way would be to modify the pool management daemon to not think in terms of username/password, but, as discussed, this has not been done. The daemon has instead been fooled into passing the address as username, a great worse-is-better solution, that solves the problem, without breaking anything, a brilliant kludge that does not need to be fixed.
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.