"most of the freeware/shareware is good for lead sheets, but doesn't offer all of the options necessary to do an orchestral or band score--stuff like the capacity to do the conductors score, with multiple staves, and the ability to extract parts in the appropriate keys"
... and ...
"ABC is not as standard a standard as you might wish"

The ABC Plus standard ties down specific extensions to the original ABC standard. Read the excellent PDF document you can download from there, written by Guido Gonzato.

abcm2ps (a practical implementation of ABC Plus, same link) handles multiple staves according to the ABCPlus standard, and abc2prt (same site again) extracts parts from multi-part scores.

There's also abcpp, a pre-processor which can automate conversion between different non-standard dialects of ABC as well as do many other things, but unlike abcm2ps I can't give a direct recommendation because I've never used it.

All those programs are free.

Anahata