Yeah, I can't see a choice override for an individual choice number being feasible: in order to figure out the choice number, you'd pretty much always have to fetch the page in the first place (for the script writer, this is essentially the initial visit_url()).
A few relevant threads to this (side) discussion:
http://kolmafia.us/showthread.php?16505 -- a recent feature request for choice override handling.
http://kolmafia.us/showthread.php?11264 -- a relay script that modularizes choice overrides (it actually does fairly well, iirc, but I also remember it cluttering the CLI with a bunch of debug messages); we'd need wider adoption from script writers, though.
A few relevant threads to this (side) discussion:
http://kolmafia.us/showthread.php?16505 -- a recent feature request for choice override handling.
http://kolmafia.us/showthread.php?11264 -- a relay script that modularizes choice overrides (it actually does fairly well, iirc, but I also remember it cluttering the CLI with a bunch of debug messages); we'd need wider adoption from script writers, though.