I think the only way to make this work is to move the troops within the constraints of the current game settings.
Example: You have a city on the frontline that loses its population in battle. You can re-station troops from towns from back in your safe zone but only to the value of the population of that town and troops would have to travel with transports which are also counted in the population.
Mythical units cannot be relocated, only conventional troops and transports and war boats.
To relocate the troops you will have to spend either 1 culture point or perhaps a value of BP (Preferred)
You can billet the empty city from multiple towns but you cannot exceed the population of the town and the population from the sending town is released after the troops are billeted into the new town.
Perhaps you could impose a timer on the billeting process (4hrs) which can be sped up with gold?
I like this idea provided the receiving city has farmspace. I agree it could be abused if done constantly but what about making it a token spell that can be used once. So if you have a frontline city and all the troops die you can relocate a full nuke there but only if you have the spell.