Why the need for such a trial? If you don't have the requested level available just tell us to stay at the flightplan level and we'll have to put up with it.
The source of the problem to me seems that controllers allow level changes. Pilots will request a different level to optimise fuel economy based on the actual conditions (wind, aircraft weight etc.) which can change rapidly between the planning stage and execution of the flight. If you can't allow it because you might overload the next sector: just say no. How difficult is that?
The solution is better coordination between sectors, IMHO...