100Hz is not supported by too many monitors, especially not TFTs. The next logical step would be to triple the pixel clock, resulting in 75Hz output for PAL. Indivision AGA MK2 will have enough memory performance to do that - and it has the exact same effect that you desire: Having the VBlanks synced.
Awesome! I like to keep one Amiga connected to a CRT for certain games and demos, needless to say 50Hz on a 2000's era CRT (short phosphor persistence) is absolutely devastating on the eyes. 75Hz will be *much* more tolerable... looks like I can finally lug the ol' Eizo Diamondtron back out of the garage.