To answer my own question for anyone who's curious:

The bios will be happy with just the Z80. No ym2610 is needed.

> The neogeo bios does a test of the Z80, and hangs with an error message if the
> Z80 behaves improperly.
>
> How much of the Z80 needs to be emulated to pass this test? Will the Z80 pass
> on its own (with the ym2610 stubbed out), or does it need parts of the ym2610 as
> well?
>