He can't move quickly, so I want the bard to be riding either on the shoulders or the head of his barbarian cohort, firing crossbow bolts from there. I figured the closest thing to this would be riding a nonstandard mount (though obviously not steering it). This is fine, according to the Ride skill: a DC 5 to stay onboard, at a -5 penalty for a nonstandard mount, if the barbarian PC does anything erratic.
But what counts as erratic? Any time the barbarian attacks? And do the halfling and the barbarian act together? If the barbarian fails a reflex save versus a spell, for instance, does the halfling automatically fail also?
These are just the questions that come up off the top of my head; hopefully you can also raise some other points that might help us figure out how to do this fairly. And just to note: I understand some of you, as DMs, wouldn't even allow this. But my DM thinks it's a quirky idea, so that's a nonissue for now. :P