Thinking of this very same dilemma, I came up with this idea using bc/cc
lay down your flame color(s)
mask "in" your flames (flames covered)
lay down the black base
remove your flame masking
spray 2 wet coats of matting clear or clear with matt additive
let it dry
come back and mask "out" your flames (flames exposed)
scuff just the flames with gray scotchbright or a fine grit sandpaper
then 2 wet coats of flow clear.
Unmask, Use a mini-gun or airbrush to wet the edge of the flames with reducer to "melt" the edge.
The rest of the car will be under the matt clear, but the flames will be glossy because they were retoped with gloss clear.....there will be no edge at all after the melting with reducer, but you could always pinstripe anyways.