The solution is better coordination between sectors, IMHO...
Yes, with proper coordination you could arrange a different flight level, but it is relative. For example a flight from LEMG to EKCH with filed level 330, asks for 390, how far down the route can you coordinate? What if there is no problem till Paris but Paris will be overloaded at the upper sector?
It is possible to check the restriction with a phone call to the flow and see if the change in level or even route would cause any problem, but longer the flight longer time this will take and how many calls can you afford to do meanwhile trying to separate traffic.
I personally think better distribution of information and free route sectors would greatly help. If pilots could file the entry and exit points into sector blocks (off military times at least), and could check somehow the restrictions on the sectors they will fly through and adjust there would be less problem with wrong predictions. This is what CFMU is already doing more or less but relying on old data. Pilots have the latest data so maybe it is time they are more involved in the loop.