This is probably totally unhelpful, but ScummVM is open source and it already has a screenshot feature. If you know C++, you could make it save every frame and put them into an AVI file.
I don't know of any third-party programs that do this, unfortunately.
Sorry, I jest. No, I don't know C++ or indeed any sort of programming language. I don't know why FRAPS doesn't work in it, so I'll need an alternative...
On the ScummVM forums, I saw a thread that mentioned DOSBox was capable of extracting video from ScummVM. There was another thread that mentioned the use of Hypercam.