Maybe it could work the way SotT’s A Cluster of Radiants works?
You get 1 free ult charge instead of it filling your ult bar. The effect would not stack and would expire after 10s (5s for Concoction, 15 for Decanter), if you ult during that time it will consume the stack early and won’t affect your purple bar at all, which will be refilling as normal in the background (or stay full if it was).
OE would get a set amount of extra ult charge at the start that can exceed his usual maximum F ‘ammo’ (but he obviously won’t be able to maintain his F ammo over the maximum). How much ult ammo could then be set by FS to a reasonable amount (imo, lower than what he gets atm from his purple and all-fruits juice which allow nonstop firing from empty for 15/10/5s) and would naturally scale down with Concoction (-50%) and up with Decanter (+50%).
This gives an incentive to use purples on IB etc. too sometimes, as an IB ult is more valuable in some situations than a single, solitary damage ult, but not always. Instead of what we have atm where purple pots almost always go to burst damage dealers or knockback spammers. It’d also still be useful on OE and at the very least easier to (re)balance on him.