You know, this just goes to show how new I am still: I never thought of this and had never read it. That is definitely a part of the problem, as the party diversity issue only gets out of hand with capped toons.
I like Lizardgrad89s idea on the mana penalty, but I think it would have to be more severe. 50% mana wouldn't stop people from leaving, just make them leave more often I fear.
This shouldn't be too hard to implement programmatically: if a character has an active instance, mana will not recharge at a tavern. Seems as simple as
open instance = true, no mana recharge
open instance = false, mana recharge
The mana pots will still be an issue. They would have to give out plat refunds for those or something. And, of course, then there will be the simultaneous cry of "NERF" by all the solo enthusiasts...oh just forget it, it will never happen even if it would improve the game experience for the average grouping player.
Mystic's solution is pretty cool, but it would need a bit of tweaking. If implemented as-is, I can see people just switching to a different toon and rejoining the party rather than taking the negative level hit.