You could boot into Mac OS to set that option, then boot back into NetBSD. As long as the latter doesn't change PRAM, it'll stick. If it does, you could determine what needs to be set in PRAM in Mac OS (save two copies, one with it set and one clear, and compare in Hex Edit), then set that in NetBSD.

Hmmm, doesn't seem to alter PRAM on my machine. Maybe it's that other NVRAM that is separate. Argh.