Never had the need for the high quality design the users of those sites provide, yet :) But even if there isn't an automated process to manage requests, i guess that (1)selecting a few of them, (2)sending a customized project/request description and (3)asking for a quote shouldn't be too much of an overhead, you have to do the most time consuming task (2) even if you decide to use 99designs. The main difference between the two approach will be the price, btw.