I understand your points but prefer to approach the problem from the other way, namely adapting the custom control in such way that the Compare Validator works with it.
The reason for this is that all controls and validators are dynamically created based on the fields and the datatype that the database expects.
I think your solution is feasible but since the Compare Validator works server side, I have the feeling that it should be possible to make it work client side as well.
but when i try to apply the same Required Field Validator to my custom control it is not possible.
The property "Control To Validate" does not show the object id of my custom control.