I actually found a very, very, very dirty trick. But my map is already so heavily modded that it could do the job. Sharing just in case 😉
I use the repairPlayers step option, which allows a player to repair for another player. The feature seems to be working ONLY if the main player has anything to repair for himself (which isn't very logic or intuitive). To say it another way, if you want Germans to be able to repare italian factories, you need to have at least one german damaged facility as well, otherwise the phase is skipped.
So I created two extra (hidden) players, " Axis" and "Allies". Only those two have the repair frontier. In order to have the repairPlayers feature working (see above), both have a (hidden) fake damaged unit on the map (in Sahara and Himalaya, respectively) that cannot be repaired (no repair frontier).
This way, the repair screen doesn't pop-up before regular players purchase phase (as they don't have the frontier). Instead, just after the Place phase of each players, there is a separate "repair" phase, actually lead by the "Axis" and "Allies" players, and using pooled resources.
This all just to make sure factories cannot be repaired before placing units...
