Don't the current rules pretty much exclude glider flights over the canyon,
even above the rim? It doesn't look like you can get below 11,500 MSL or so,
and not at all through the no-fly areas. Unless gliders are special (which
they could be, since I think the main motivation for the SFRA is noise).

No. The special air traffic rules specify 14500 MSL as the lowest you
can go. That gives a VFR window up to 17999, or higher with clearance
from Center.

-ted