A Mac DA-15 to VGA adapter is entirely passive? [...]This might help <https://www.bigmessowires.com/2023/10/04/classic-macintosh-video-signals-demystified-designing-a-mac-to-vga-adapter-with-lm1881/>.Oof! There is clearly a lot more going on here than I'd realized.
If you figure out that the machine boots, or at least tries to, you can bend a piece of paper clip to make the Mac think it has a monitor. Are you getting a chime when you power it on?
Did you mention BlueSCSI? I might be able to make a disk image of a bootable Mac HFS partition along with Booter that launches automatically and boots a NetBSD kernel in to a clean NetBSD installation. Let me know if that'd help.
John