I don't see why not; units can be gifted freely as long as they're not being used for Heroic Epic purposes. If Dantski wants to pay the full freight for upgrading the muskets, walk them into Sullla's territory, gift them, have them upgrade & sit there for a turn, and then come back, he's welcome to it: He'd be paying a pretty steep price for the rifles.
The city gifting was abusable because it let each of several team members use their traits for half-cost buildings, not to mention the power of passing certain wonders around (MoM being the most absurdly abusable of all of them).
The city gifting was abusable because it let each of several team members use their traits for half-cost buildings, not to mention the power of passing certain wonders around (MoM being the most absurdly abusable of all of them).