Diagnostics are meant to diagnose what is the status of the operational aspects of the car - specifically the engine. Whether it is working OK, or whether there are some parameter values which need attention for proper working.
I do not see how the odometer OR the current fuel tank reading could have any bearing on a diagnostic. The odometer might - in that if the car had gone 500000 miles and some parameters were out of whack, then the diagnostician might throw up his hands and just say "oh well, this vehicle has run its course".
HOWEVER - as a solution to your dilemna - since both odometer AND fuel level are readily visible from the drivers seat, and usually also quite static during any diagnostic run, maybe you can just manually feed in these numbers and not worry about "legislation" ? And how to change the value ... which IS illegal ?