xsnrg wrote: If the autofocus routine uses the offset as a starting point for the auto-focus run, that would be wonderful though.

Hmm, but it's doing that, isn't it? At least for me, the offset is applied as soon as the filter changes. Then the AF is triggered, starting from that setting.
(I have AF on filter change set for all filters, and the respective relative offsets in the table).

As for NB, it might help to use near-by BB filters. I.e., for me the focus for Hα and R is basically identical. I don't have OIII, but could imagine its focus would be very similar to G? (At least if the filters are from the same manufacturer and have the same thickness). Then do a 'fake' BB exposure to trigger AF (AF on filter change set), then switch to NB (set same offset, no AF on change).

I myself do AF on Hα, but I do have a 6nm one. That is enough to get sufficient signal in 8s exposures (ASI1600 at unity gain).