Thanks to turska for pointing this out...
The latest x264vfw, released two months ago, adds 4:2:2 and 4:4:4 support, with the gbr hack. Combined with zero-latency mode and lossless encoding, this means that it can be used for TASVideos encoding purposes (ie, true lossless). Filesizes are quite small, and speed is quite fast, as expected with x264. However, you have to be careful with your VFW codec stack, as quite a few things can bid to decode it, and many do so incorrectly.
In order to get full lossless decoding, you should have x264vfw itself do the decoding, and you must tell AviSource to request the right pixel type from it. Otherwise, AviSource (in 2.60) asks for YV24, which the decoder happily supplies (doing lossy colorspace conversions along the way).
Language: avisynth
AviSource ("checksum-x264.avi", pixel_type = "RGB32")
I have tested this and it passed a checksum for exactly lossless.