What are the differences?
"except that" should be followed by a clause.
How about "except for"? Can it be followed by a present participle?
Thanks in advance.

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?

Thanks Ron.
It doesn't come from any OG but a source that I am not supposed to discuss here or anywhere else ;-)
But I *think* that it is okay to discuss the concept ;-)

It's always okay to discuss concepts :)