You can append the FRAPS files with AviSynth and then run it through MeGUI.
AviSource("filepath") +\
AviSource("filepath2") +\
...
...
...
...
AviSource("filepath9000")
I did this with Parasite Eve and Maverick Hunter X.
That way, you won't have to wait to make one gigantic file.
But if you don't want to do that, I say use ffdshow to make your gigantic file (set VFW encoder thingy to H.264 lossless.)
I find it less of a hassle to run the video through VirtualDub to get your giga file and then wait for an encode in MeGUI, when you can just append all the clips in AviSynth and then run the files through MeGUI.
MP4 is just a container, just like AVI and MKV. Whether it is MKV or MP4, the quality will still be the same. (don't know if I am wrong, and I really don't care either.)