I'm not sure exactly what you're looking to do,
but another thing which I do for local modifications/settings is this:

-- Create a new file with makefile rules which check to see if the
package is being built (see devel/cpuflags/optimize_gcc.mk for a good
way to do this properly) and/or buildlinked in ( you can do .if
!empty(PKG_BUILDLINK3_MK:U:M+) ... for the latter) and set
CONFIGURE_ARGS, etc. as well as use the SUBST framework to make local
changes.  Hope this helps.  I find using SUBST for changes especially
helps for working on different Mesa versions, because I don't have to
keep making new patches.  Moreover, if you have a consistent set of
changes which you apply over a large number of files, it will find
what has to be changed for you.  Then you can maintain a patch set on
top of that.

Regards,

--Blair