you can't use a
present participle in that sort of situation, but you can use a
gerund. the two are easily confused, because they look the same - both manifest as the -ing form - but the gerund is the version that's being used as a noun or noun equivalent.
in general, 'except for' can only be used with
nouns, or with things that are equivalent to nouns.
--
here are a couple of outside threads i found on this topic. i think 'except' and 'except for' are, for the most part, fungible, but the real issue is the way in which the gmat uses them.
http://www.english-test.net/forum/ftopic460.html
http://forum.wordreference.com/showthread.php?t=329277
have you come across 'except' vs. 'except for' in any gmat problems? if so, does the official guide express any favoritism for one over the other?