FL
I find your analysis hard to believe in entirety and it is certainly flawed in principle. In reality the SFH cost includes pretty much all the indirect costs of operating an aircraft such as all calendar maintenance, depreciation and insurance - operators are certainly going to build these in or it wouldn't be commercially viable. What's more the cost includes profit - for them. I do of course appreciate that the direct cost to the hirer is what he/she has to pay though and that this is what matters.
With regard to owner-pilot aircraft on the other hand I would strongly argue that the majority of maintenance is a direct cost of operating the aircraft and can be reasonably appportioned on an hourly basis and costed to a given flight. With increasing uptake of PBH/SBH schemes for engines and other major components direct costs are clearly set. So would you say such contracted hourly SBH costs can not be included? I do however accept that turbine aircraft are much less likely to be the subject of cost sharing but there must be some and the principle is very important here to avoid breaking the law. My Robinson knowledge is limited but I recall that there is factory set total rebuild cost every ?2000 hours, for example, surely making a fair direct cost easy to establish.
If your interpretation is correct and someonebody wanted to make extensive use of cost sharing then perhaps the best thing would be for owners of aircraft to establish a structure by which they would effectively rent it to themselves at a market rate which then forms the cost to be shared!