I dont think that the game should limit the tax level. Taxes are the king's prerogative and so the players should be free to do what they want. But revolts should be much more common ('A good shepherd should shear his flock, not skin it' said the Roman saying).
What about the following : taxes are not limited but when unrest reaches 100, there is a chance every season that the province openly rebel. The chance is (unrest level -100) %. Or 10% for 110 unrest, and 100% for 200 unrest. If the insurrection happens you see 30 indep units (or 1/1000 pop ?) appearing from nowhere to free the province. This solution is both logical and realistic : dont you think that local population could revolt if taxes are killing them, if pillage is allowed or if young virgins are abducted by evil mages ?
Cheers