I don't think you need to mask anything if you have all the threads chased, BB and headtube faced and seat tube reamed, which is what you should do after any re-paint. The powdercoater is not going to paint the threads, although there will be some overspray, which will easily get cleaned up with the frame prep after powdercoating.
Also suggest bead blasting to avoid rust.
__________________
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1