I think you're missing the point - he doesn't want a quote or anything like that... he just wants the exchange to figure out the appropriate order size to deplete a given amount of funds. Whether the orders fill or not, I don't think anyone in support of this idea really gives a shit.
FWIW, this is only really needed on TradeHill et all - on MtGox as long as you shoot the moon on your quantity, it'll figure it out for you... it'll make two orders, one which can fill and one which can't because of insufficient funds. If you wanna buy $100 worth of BTC at $13, simply put say 10BTC in the quantity field, then cancel the order which isn't actually on the books because of insufficient funds.
On TH though, it's a pain in the arse. Figuring out how much BTC to buy to deplete a given amount of USD either involves a calculator or brute-force guessing.
This.
I'm mystified that the other exchanges haven't figured this out. I don't think mtgox maintains it's volume lead purely on momentum. It's about convenience too.
The MtGox interface could be slicker for sure - but it's core ability to place pending orders without the covering funds is handy both for the auto-calculation mentioned above, and for placing a trade which will only run if some other order or deposit supplies the funds.