I have topped up before, yes, both in AG and partial mash / extract scenarios. But...those are to reach volume targets.
Personally I think your plan of adding water as you go would work, and the best way to do find out is to brew a batch and see if you get the bitterness you're seeking.
If it doesn't, well, there's a good excuse for a turkey fryer and large pot or keggle combo so you can eliminate the problem once and for all.
We cannot defend freedom abroad by deserting it at home. Edward R. Murrow