I was considering doing that as well, but I need to get my system up and running first...
The problem with doing this on Gentoo is that LinuxMCE was designed for Ubuntu and so most, if not all the, auto-configuration would have to be modified to use Gentoo's config files and scripts. Of course you will still have the issues of compiling using different versions of many libraries and programs. This will definitly be more than a small effort...
The first and relatively easy ebuild to create would be the orbiter. Tackling the whole Media Director would be a chore especially considering that LinuxMCE does various sanity checks and monitoring that would have to be modified to support Gentoo. Once that's done doing the core will be easier (mainly because the bulk of the hard stuff was just done), but still a rather large effort. Of course the core should be on Gentoo Hardened, which will have it's own set of issues.