Since the founders are experts at genetic engineering, I'm pretty sure it wouldn't be too difficult to just make the Jem'Hadar and Vorta immune to assimilation if they aren't already. If the doctor from Voyager could do the trick, im sure the Founders could do the same for their DNA. I mean they made "adjustments" to the Jem'Hadar to fight in the Alpha Quadrant, etc. All it would take is a simple resequence and the Borg lose that advantage instantly.
The Doctor never made humans impervious to assimilation.
On the contrary, what he did was conjure up a neural suppressant which lasted temporarily (even less for Tuvok) that prevented the away team consciences from being slaved up by the collective ... their bodies on the other hand were still assimilated.
It was a temporary measure that would eventually wore off (it should have wore off faster though).
The Founders wouldn't be able to make the Jem'Hadaar impervious to assimilation.
Temporarily it might do the trick to alter their DNA, however, the Borg would find a way around it on their own eventually because the nanpoprobes adapt and rewrite the DNA.