Is it possible to resupply off-map artillery?

Solution 1:

I don't believe this to be currently possible. Nor in future. Here's why.

The current player is in command of given battle-group and player can influence the scenario outcome by giving orders related to map. Even the off-map units are "called" to map whether by observation units or to the map-side (planes). Every unit has explicitly limited supply (which is realistic and intuitive) and the only supply replenishment tool is also an on map unit. The player has no way to send supply trucks to stand next to off-map artillery (or ships :P).

If this supply gain is not automatic then including such feature would require player to divert their attention from on-map action to off-map micromanagement. I don't believe the designers want to make that decision. And that' another game - Hearts of Iron ;)

Also, I would consider any time-based automatic "magic" reload a bad solution (which is perhaps why designers have discarded this as well?), as the barrages are already very powerful and you usually get not one but ~3 of them. Getting constant bombardment would be unrealistic, OP and possibly break MP.