Lagarith: http://lags.leetcode.net/codec.html
Dxtory: http://exkode.com/dxtory-features-en.html
I believe I made a thread with such feature request (lagarith) 3 years ago. Can't find it, though.
Anyway, the request is still relevant and not just for me, I believe. Avidemux can't natively read/open FRAPS video but there is still no support for Dxtory and Lagarith. While the first one is already quite popular (quite an awesome piece of software), the second one is still best option to render lossless video of relatively small size (much lesser then raw or Huffyuv or any analogues). There is no reason to add write support, of course, only reading (in case with Lagarith it should be more easily since it provides source code).
P.S. yes, I know about AVS proxy workaround, but it has its own issues.