For starters, it would take the lawyers a decade to sort out how that would fit in with the Chicago Convention, if at all.
Secondly, as has to be asked with any PFI, you would need enough redundancy so as not to endanger charter ops for ad hoc mil ops. That sort of redundancy is unjustifiable in fleet-size terms these days. How would you insure against losing charter tasks when mil ops take precedence? Could you compete with genuine charter ops without that uncertainty?
If you're serious about using them for combat ops and not just Strat, then RoE enter the fray. Not for the crews but for those carried onboard for protection. (Read Blackwater for a good explanation of the dilemmas of merc ops!)
The chance of varied flying would appeal to many at the moment though!