x264 recently introduced 10bit capability, but isn't 16bit capable yet (from my understanding, apparently 16bit would need a somewhat major rewrite of parts of the code, whereas 10bit only had minimal differences, so that's why). If you're fine with the improvement from 8bit to 10bit, you can get a build from here:http://forum.doom9.org/showthread.php?p=1414996#post1414996
However a warning: that's r1666, we're now on r1680. No one bothered making 9bit and 10bit builds after r1666. And there is a reason: there aren't enough decoders that can support those bitdepths. DivX7 seems to be the only H.264 decoder that can support it, and you can't quite count on people to have it. Also, keep in mind that the higher bitdepths takes more processing power both during encoding and decoding.
So imho, if you really want to make an 10bit encode, then you should probably also offer an 8bit encode too regardless. I don't think it's quite the time for this step yet.
I guess having a dithered encode is the best way if the gradients can't hold up and become a banded mess.
EDIT: BTW, might I ask which source are you working with? Most anime is only worthy of 720p at most as far as actual detail goes, despite the m2ts being 1080p (or 1080i)... so chances are you could just edit at 720p and not mind.