You have described a $20K repair on a product
which might not be worth $20K right after the
repair. My suggestion for the rods was to try
a $2K or less repair which would allow current
users the peace of mind to continue flying their
gliders.
My understanding of the DG 300 is that
it has a very stiff wing, presumably even with
undulations in the spar caps. Since Graphlite
ships in two and three ft. dia. spools it must be
plenty flexible and it is far stronger than the
equivalent amount of fiberglass rovings. My
suggestion would be to saw several kerfs of
varying lengths and depths through the undulation
area extending for some length on either side of
the area possibly to the end of the spar stubs.
Graphlite rods could be epoxied into the kerfs,
like rebar in concrete, and they would take the
load from the rovings they butt up against.
I suppose fiberglass cloth could be wrapped and
epoxied around the spar butt to prevent the rods
from popping out, if that could happen, but I think
you would have a much stronger than designed spar
with a very stiff wing. I have heard of someone cutting
a kerf in wooden spars and putting in the Graphlite
rods
to improve the strength.