Hi!
I am not sure there is a poilicy, but Gentoo does make non-free software available in the same ways as the free software. Feel free to package it and host it in the overlay. However, you should take the following in consideration when packaging it:
1. Make sure the package has a valid license. You should get a license from the homepage or the author and put it in the licenses/ directory in the overlay. After that it is just to specify the license in LICENSE="some-license"
2. Pre-compiled packages should go into /opt.
3. You must not allow the package manager to download the application if that is disallowed by the author. You can handle that with RESTRICT="fetch", if you need to.
Those are the things I can think of now, but I might have missed something.
Cheers,
Karl