Just my two cents...
In AACW i think the instant redeployment is Ok as it is now as it's limited to special units like leaders and special support units that appear in the capital by default (game engine limitation) but in real life could have appeared anywhere.
If some work would be done (not top priority IMHO) in the case of this "special" kind of units, placing them when thy first appear on a general off map box (like a reserve) and letting the players choose where to deploy them instantly (limited to cities maybe) would be cool.
If we are talking of a future game (VGN??) where you could redeploy full combat units (regiments, divisions, corps...) like in HOI, it would be needed the x turns out of map before repapering for realism sake.
I think the best option for this kind of combat force redeployment would be an "In transit" out of map box or at least a page on the ledger where you could see the units on redeployment and the expected arrival time.
To use a system that leaves the units locked in place for some turns (on origin or destination) can be confusing for the player. And besides, have bad effects like units in redeployment being attacked destroyed while immobile (on a place where they should not be in reality).
In the case of AACW i don't think a combat force redeployment is necessary at all (river and naval pools work great for this). But probably in a global game it would easy force management (like sending a regiment form London to colonial India with out having to drag and drop it half a world away.
Regards!