I have thought of this one for some time also. This would be the most realistic reinforcement option in my mind. Unlimited ie.transferring troops across the world in one turn is unrealistic. Chained is much the same, but limited to one group of troops. Adjacent is realistic in the space it covers, but unrealistic that you cannot move more than one group. Unlimited adjacent would be the correct way to say it, and would be the most the realistic i.e. move as many groups as you want, but only one space.
Real time risk, like Risk 2 could also be interesting. Pick your place(s) of battle, and decide how many troops to commit, realizing that your opponent(s) is also doing the same. After everyone decides, the battle(s) is executed autonmatically, and you are unable to call back troops if your battle doesn't quite your way. The reinforcing process would be a turn all in itself, followed by the next round of realtime risk, and so on.