Apple's DNS is just a gui on top of BIND, the most popular (or, at least, common) DNS server out there.
There's nothing you can't do in DNS. The only limitation is what Apple's GUI lets you do.
Off hand I can't recall if Apple'S GUI lets you define arbitrary TXT and SRV records - SRV is also used by bonjour, so it should, but that might just be handed behind the scenes. In any case, you're perfectly at liberty to edit the zone files directly (they're just text files) to add whatever records you like.
Irritatingly, Apple have moved the location of the zone files across various versions of Mac OS X Server and I don't recall where they are in 10.6.x, but start at /etc/named.conf and look for the various directory and/or include directives.