Why not register independent so you could vote in either primary?
MAGOP is decent, although of course I strongly prefer the (cosmo-)libertarian Weld/Baker wing over the moderate-to-liberal Romney/Brown wing.
I'm involved in my town's local politics, (well, somewhat) and signed up with my town's local affiliate of the GOP. I have to be a registered Republican to vote on committee decisions - which makes me much more influential than voting in a Democratic primary - not to mention that it's a matter of principle that I don't vote for a primary candidate unless I would vote them in the general...
...and I sure as
hell don't plan on voting for any Democrats in the future.