How does 175 kbit 8bit DNxHD (free codec from avid site) look? Or do you have a mac system you could use to convert directly from lossless to prores 422 or 4444? Prores may be your only option. If the end result is bound for web anyway, you are probably out of luck. Youtube or even vimeo are going to crush the grain sharpness quality on their end no matter what you upload. I have struggled with this in the past. High quality grain just does not compress very well - it's the first victim of compression.
Or you could simply lower your standards...