It seems I was mistaken about the native 10bit support for AE as it only has the following increments 8/16/32. Premiere Pro CS5+ however can do native 10bit, which is what I probably confused it with. What I'd recommend is working in 32bit [even though your display is only 8bit btw] then once you are done with the project and exported it as such. Simply scale it down to 10bit when compressing it to a 10bit h264 file.
Your monitor can only display 8bit color as is unless you have a true color display. So while you're getting a little more room to play with with ramping and what not, going beyond 10bit for final export is pointless.
/edit- Because we're not certain about a 32bit pipe solution for h264 the following will mediate that.
Make your comp in 32bit and do what you want to do. Pre-compose the 32bit composition into a 16bit one and let AE dither it down to 16bit. Thus avoiding having the plugin to try and run in 16bit, we'll let AE take care of that. Export that composition as the 16bit comp and pipe that into a 16bit compatible h264 encoding pipeline. Encode that puppy,Bob's your uncle! You've now got a juicy 10bit h264 encode, provided you know how to encode in 10bit h264 of course.
//edit- Beep boop your 16 to 10 pipe solution: viewtopic.php?f=46&t=108096#p1427025
Skip avs and load straight into x264 10bit otherwise it'll be dithered down to 8bit. Thus making all your work pointless.
PS Mirko is a pizza-hut lovin' Italian.