Englishal, if everything you say here is open and said to a prospective buyer of a share, then that's a perfectly legitimate way (legally and morally) of running the group. The buyer knows what he/she's getting into, and can expect bills (possibly rather large bills) above and beyond the monthly fee and wet rate. Setting up your own private engine fund is a good idea then if your personal financial situation is tight.
Personally, if the aircraft required an "annual" instead of a "100 hour check" I would make sure the annual would be covered by an uplift in the montly fee, instead of an uplift in the wet rate. But that's just me and my personal opinion on how to make "fixed" cost "variable"...