I have not found a way of withdrawing completely from a territory (i.e., reduce my armies to zero) so that an ally can take over the bonus. In particular, although I can reduce my occupation force to one and then tell my ally to go ahead and attack to take over a bonus, AI allies don't seem to take the hint and I have no way that I know of of telling them to do so. Are there any work-arounds for this? Or is simply a matter of never enter a territory that you want an AI ally to take over because once you're in, you're stuck unless an enemy takes it?

No, there is no way to do that currently.

Or use a gift card

If the bonus is all 1s, and all owned by your team, and if the AI has more then 50% of the bonus, and the AI does not have anywhere else to expand and has sufficient armies, they will capture it. It's typically much more efficient to take it yourself, but it is possible to get the AI to take it.