This sounds tricky, I must have a good look at a 27.
I am not sure how to
get mylar to cover the gap but not bend when the surfaces
are moved.

Short strips of mylar are secured to the control surface
in order to cover the hinge gaps, then the wing mounted
mylar rubs on the control mylar......look at a 27.

Maybe we should just use hinge tape on its own. Lots
of work to replace
every season, but possibly as effective as the other
options.

Teflon gap tape will last for several years. Be sure
to install with control in fully deflected position
+ a tad of a dimple into the hinge line or shrinkage
will restrict the movement.
Cheers,
JJ