Installation went pretty fast since I'm surrounded of Pentium 4 computers running Gentoo at work (providing binary packages), but basically I entered the myth* ebuilds into /etc/portage/package.keywords and started merging.
Glibc with NPTL, udev, sane USE-flags (will publish in Summary).

Then I set up an order to get everything working:

1. Get it to boot
2. Configure X
3. Configure MythTV and plugins
4. Enable sound
5. Get the remote to work
6. Control temperature
7. Get TV-in to work
8. Get Radio-in to work