Might not be NAR approved, but would not this whole excersise be accomplished with a length of good old green cannon fuse ?

run an aluminum tube up into the birtd - just like guys use for dethermalizer fuse... possibly have the fuse into the flame stream at IGNITION, so it is sure to light - cannon fuse burns same speed whether contained or not ( unlike thermalite - which also needs an LEUP ).

A small LPR rocket only needs maybe 1/4 gram of BP for ejection, contained in a tape wrap at end of fues ( make it up and feed into tube from top, then tape to motor at bottom .

Sort of like the old mid-power composite 2 stagers ( I think Jerry called it 'discontinious staging" .

worked well if timed right.

~ scientific Al