One easy option to implement would be to have hirelings/supporters as a Player Character sans any Encounter or Daily powers (and probably a limited amount of utility powers as well) Keeping all else the same it should be easy enough to run a character like that and you wont have to do a lot of rebalancing.

Another option could be to say that all of the players characters have a limited amount of Encounter/Daily/Utility powers. This would make characters feel more equal and be less of a loss opposed to a 'main' character death.

Either way just work to constrain player choice down to a level that your least flexible player can manage within a time that does not bother you/the other players.