We also have a substantial engine fund that does not feature in share value.
It depends on what you define as "share value". But if you define "share value" as the amount of money I need to pay you to take over the ownership of your share, then I think it should be included. After all, if I bought all shares that way, I also become the owner of the engine fund, don't I?
Look at the aircraft "group" as a limited company. It's got assets, primarily the aircraft but also any cash reserves, spare parts, fuel in the tank, can of oil in the drawer, maybe a prime spot in a hangar that others would be willing to pay money for, or a lucrative and rare contract for cheap landings. All these assets should feature on your virtual "balance sheet". On the other side, you also might have liabilities. A loan perhaps, or maybe members have deposited money for flying in advance in the groups bank account. These should be deducted from the groups assets. And whatever is at the bottom of that balance sheet is what the total "group" is worth. Divide by the number of shares and you get the fair share price.
Obviously the airframe itself, in the current market, is very hard to value and that makes the share price hard to value. So at the end of the day there is still a matter of finding someone who is willing to believe your balance sheet, and actually put down that amount of money. But that doesn't mean that for the rest you shouldn't be following sound accounting principles.