I spent some time on making x264 support vpy scripts internally. Patch
here.
The ideas and some code taken from ffmpeg demuxer proposed here, avs.c input from x264 and NVEnc's vpy input module.
Not sure if that's faster than piping, but looks like complex scripts uses all CPU cores.
Code will definitely compile well with icl 2018 or gcc 7.3.0. I didn't tried another compilers.