I can think of very few aeroplanes where, if it was considered important enough, a small panel / filler-cap where a dipstick could be inserted into the deepest part of the tank couldn't be inserted with very little trouble at-all. Also a reasonably cheap totaliser could be inserted in the main fuel line very easily with a cockpit readout - so long as you know how much fuel you had to start with there's no particular problem. The CAA mod fees would probably be more than the device itself.
Incidentally, in a previous life I used to do flight testing on a British light twin called a Jaguar. That had an obscene number of tanks, and a largely automated fuel system. Fuel gauging was through a totaliser which worked on everything except the tailtank. Drill was not to take-off until it started going down because until it had, you were out of aft CG. If you've ever seen a Jag sat at the start of a runway for longer than seems sensible in reheat before taking the brakes off, that's probably why!
G