I honestly like this idea, especially since regular tanks are restricted to the main roads.
Ways to eliminate a Juggernaut:
Special (i.e. different color paint) paint for heavy gunners or engineers that only takes out juggernauts
different color first strike rounds
just a few things you could use to deal with a unstoppable juggernaut.
limit the juggernauts time on the field to 15mins or an elimination, whichever comes first.
Mandate that regular paintball hits do not count. Now while some people are still gonna shoot at him, most aren't going to waist paint shooting him if it doesn't do anything.
Anybody that would want to run as a juggernaut would of course have to be FULLY padded and be required to do a gauntlet run to make sure they can deal with getting lit up.