It may be what you've already found, but there's this page:
http://amc.maa.org/whatswhat.shtml [oops, I typed too slow ;-)]
I don't think there's much in the way of officially Olympiad-aimed programs except for people who have already done well in the AMC10/12 and AIME contests, but after that there's the MOSP, which is residential, and at which further tests determine (or contribute strongly to determining) the USA team for the IMO.
One possibly-useful source of information about other summer schools etc. might be
http://www.artofproblemsolving.com/Forum/viewforum.php?f=215
