I actually ended up emailing him using the contact details on the repo, and got a very fast reply.
He's saying he has tried it earlier, but that it is not an easy task because the spotlight feature uses the Gnome Tracker Project for indexing, and that is deeply tied to Linux and uses some first class Linux libraries.
According to Steffen, it is not impossible, but requires a lot of work. Something that might not be justified since Apple has changed over to Samba4 and main netatalk developers closed their company to work on samba.
So I guess the way to go is to switch over from AFP to Samba4, but
as far as I can tell, Samba4 uses the same Gnome Tracker Project to index files – so it could be I'm out of luck. I don't think I am "advanced enough" to figure out how to get that working.