I just got an email from Tom @ mybitcoin. He said that they are working on a solution for this. He told me that overpaying on a temporary address results in the remaining coins going into a "lost and found bin" right now.
He also pointed out that Bitcoin addresses are accumulators. He said that you can actually make a whole bunch of payments onto a temporary address over a 24 hour period until the total amount is equal to the target amount.
Anyway, I'm urging mybitcoin to signup on the forum here. I'm tired of relaying information.