As far as I know, you can use any of the macros defined in the source under src/macros.c.
Some of the more useful ones include: PROGRAM and FACILITY. Check out that source file for the complete list.
You might try a destination defined like $HOST/$PROGRAM.log or $HOST/$FACILITY.log. (I haven't tried this ... but it's there defined in the source, so I'd expect it to work...)
If your service doesn't give you a useful PROGRAM string but gives you other predictable things (or you just don't want a log for every service, just one for particular services), you could also define match filters. Campin.net has a bit of documentation
about it that you might be interested in checking out.