I seem to remember some application (I don't know if it was avs2yuv or not; I could almost swear there was a GUI, though) that could load 32-bit AviSynth and then pipe to 64-bit x264. It'd still be particularly bottlenecked on the loading stage, but you could at least squeeze out a performance gain from the encoder. AVFS can mix 32-bit AviSynth and 64-bit processing apps also, and it's already included in the lastest AMVapp roster.
The other option would be to use the special
64-bit AviSynth build by squid_80 but not use any filters; do all filtering beforehand and output to a lossless AVI file, then load that with 64-bit AviSynth and a simple AVISource (making sure of course that you have a 64-bit version of ffdshow or whichever codec you're using doing the decoding). That's generally what I do on 32-bit as it is. And as far as I know, 64-bit AviSynth
should work with 64-bit x264 just fine, especially for something that simple. The only downside is that it's not at version parity with the 32-bit main branch (the 64-bit build is only at 2.56, I think). I don't know, but that might cause some problems on Vista/Win7. You also have to install it manually.
Maybe I've just gotten too used to the command-line, but it's fairly easy to configure mencoder to always output raw YV12 (or I420, I can never keep straight which exact one x264 requires, as I420 and YV12 are the same except for swapped channels) and then call it with a simple
mencoder input.avs -o output.i420 (or output.yv12, as the case may be). For uncompressed I420 in AVI, you can actually just use ffdshow's VFW interface for that. Outputting to raw is necessary for those on Linux that don't want to pipe from Wine, but not for Windows users - x264 can use AVI files themselves, just so long as they're uncompressed I420 or YV12.