Now that the two patches for FreeBSD are applied on upstream sources, mesa-6.4 can build on Gentoo/FreeBSD with the trivial patch attached. Thanks, Diego
Created attachment 71867 [details, diff] mesa-freebsd.patch
Why don't you want freebsd-dri(-$ARCH)?
Mainly because atm I don't have ways to test it if it works or not.. so I played it safe for now.
Added to 6.4-r1.