The problem lies in operational control. If the operator is responsible for maintenance and scheduling, and employs the pilot, then it can be construed as operational control and they may operate under part 91.
If, however, it is a third party that is renting the airplane and they are not exercizing control over how the airplane is maintained or scheduled - then what you have is a can of worms. Now, I fully understand that such arrangements take place on a daily basis - but I also understand that legitimate operators that spend a great deal of time and resources on compliance take exception to such operations and will report them to the local FSDO.
It happened to a good friend of mine a while back, and LOI's and violations were distributed liberally.
So, if you're going to do this, please ensure that the client has some ownership interest or long-term (three months is a good minimum) lease agreement for the aircraft. If it is a turbine aircraft, you may find yourself under 91K instead of 91 - 91K is almost as restrictive as 135.
As far as maintenance goes, commercial operation will require 100 hour inspections - 91 will not (I assume we are actually discussing a piston aircraft), but bear in mind that commercial operation under 91 will only permit such operations as sightseeing, flight instruction, rental (without pilot services!), pipeline inspection, banner towing, etc..