In my experimental tree, I've hacked together a small function that parses a
string for a devclass name and unit number, and looks that up.

I'm also trying a number of other approaches; mainly I'm trying to understand
how newbus works, and what kind of driver I want at the various points, ideally
auto-attached, or configured by hints, instead of by custom code.  I think I'll
need another couple of days to get a good enough understanding of drivers,
devclasses and their tree, and the device tree.