I thought I'd read the whole thread, sorry. I wonder if the common 1/8" diam. platinum/ceramic beads, common in some catalytic converters, could play a role here. I envision a stack of these pellets (possibly even ONE pellet?) caged in the precombustion chamber with a common glow plug threaded in at the top or the side.

Taking the glow adapter concept a bit further, (A) gutting the sparkplug; (B)
fabricating the small annular cage, with stack of 1/8'' pellots up the center annulus, with the glow plug on the side. (C) fabricating the
cage withe the beads in the outer annulus, and the glow plug on top. (probably wouldn't work LOL)

Seems the igniter on the Thunder Tiger engine was quite small, smaller than those on the O-200 mounted in the Doctors' Cub. Wonder what the good doctor thinks of these plugs now, and if he is still using them?