Does anyone know of an SGF viewer/editor that will accept a move number as a command line argument along with an SGF file, and open it to that specific move number?

does gnugo? I think it'll continue playing at that move number. Not an editor/viewer.

Forcing a GTP-aware client to begin a GNU Go session from a specific SGF file at a certain move number might be an interesting hack, thanks.

In the meantime, SmartGo is the only client I have found so far that seems to be able to open to a specific move.

GoGui seems to do exactly what you want. Eg: "gogui -move 5 game.sgf". See: http://gogui.sourceforge.net/

Sorry for the late reply BTW.