I wouldn't call myself an expert. That title might go to Dan Isaacs. But one item I took note of in your workflow is that the V210 codec is for use in conjunction with the AJA card. Not having the card usually means the V210 files simply won't work.
Do you have that card? If not, have you tested this workflow, and does it actually work?
Having said all that, there is another thread discussing the use of a new intermediary codec - UT v7.0. This codec is lossless, and does have a YUV 4:2:0 mode, so your color space would not need changing at all.
I gave UT a shot. However I had it crashing few times in encoder. Then googled for alternatives and there seems to be a whole bunch of YV12 capable lossless codecs out there: Hyffuyv, Lagarith, MSU, Snow, and FFV1. I tested some of them, and Lagarith seems to do the trick for me (in Lagarith, setting "prevent upsampling" flag maybe worthwile). So I am now to: 4:2:0 - ??? - 4:2:0; that is:
1. Import H264 YUV 4:2:0 video to premiere
2. Work with the video in premiere and export using "Microsoft AVI - Lagarith lossless codec" to YV12 AVI (YUV 4:2:0)
3. Use Avisynth to frame serve AVI to x264 (only line in avisynth script is AviSource ("inputYV12.avi") )
I wonder what color space Premiere works internally to apply all the effects and everything..
Again - Thanks for making me think alternatives here!!
PS. for v210, I do not have card, but I can export in v210 from premiere. Then I can either use VirtualDubs build-in v210 codec (or YUYV had I exported to that) to convert back to something else. There is also a video for windows codec for v210 that I tried (google: drastic v210) you could use.
It's my understanding that some of the effects will operate in the native YUV space of the media, but a great many of them will only operate in RGB, doing on the fly internal conversion.