I think something like this is a much needed mechanic to make globals wars more dynamic and opens the possibilities for exciting counter-blitzes in the second or third rounds where alliances rebuild the military of tiered members over a couple days while in beige. Functionally, this could contribute to breaking the cyclical nature of large wars.
In my opinion, for this mechanic to be balanced:
Nations that receive units need to be on beige. To prevent cringe down-declares in wars and to protect piracy so that alliance counters can not instantly max out their military
The number of allowed unit transfers per day needs to correlate with the size of the alliance (whether it be score, nations, cities, average score, etc)
Alliance Reserve unit rebuys need to be very slow. 30-40 days to hit the max
Unit upkeep should be similar to normal unit upkeep and the cost should be spread across all the alliance members
Units shouldn't be sent to inactive nations that happen to have 5/5/5/3 military builds