I don't like the idea of a repair tank. If there was one it would be another vehicle that "get's in the way" and as an (assumed) light armoured vehicle it would probably be just cannon fodder, literally free VP for the opponent.
If field repairs are dying due to snipers "all the time", then the healers are doing something wrong. Most field repairs get killed due to splash damage from Arties and the recently buffed MRLS. Meds / Mammoth / even Stanks also got a considerable amount of splash damage. Another thing are sneaky stanks rolling over field repairs with "deaf" mammoth drivers not noticing them...
And yeah: SBH, Patch, Airstrikes, chem troopers, free infantry (esp. shotgunner & flametrooper) get probably more field repair kills than the avg. sniper. Sure there are exceptionnally good snipers as poi / nick / deadbeat / Minji / (others). But that's individual player skill and not a balancing issue.
Most map environments offer decent possibilities of cover. It's up to the player who's repairing tanks to i) seek cover from splash and ii) from snipers / other infantry. If the healer is not moving at all...
Next thing: the tankers. Field repair is no solo job. It's up to the tanker to protect their healers & give them decent cover with their vehicle. In my opinion it's the tankers job to get rid of infantry threats as SBH or Patch. Most Technicians fighting any of these units will most likely die. But many field repairs also panic if they are on low health and see e.g. a chemical coming around the corner... why not entering the safety of the mammoth tank you're repairing?
Esp. during public matches a lot of field repairs just focus on their tank. They don't even notice that another Tech/Hottie is repairing them. Q-spamming them with "I NEED REPAIR" is (often) also a waste of time...
BioZ / Denuvian and me are often playing together using TeamSpeak to coordinate each other. Two tankers, one healer (most of the time). If the healer is on low health with no other healer around we just swap positions. Sure that needs communication & trust. On a public server most people won't do that... But with a small community we have here most player know each other (at least they know the names..) So if you have a dedicated field repair in your team... why not offering him/her to take over your tank if they are low on health (before you toll out on the battlefield..) - as most people won't understand what you want them to do during a tank battle / siege / whatever
This may be the case if the PUG teams are unbalanced in regards of snipers. Last Saturday we had kind of that situation.
Team 2 had XEN & Ks.ol, our designated sniper had no chance against theam. Esp. during the Crash Site match a lof of field repairs got killed by XEN / Ks.ol during the first half of the match, but GDI still won. We switched tactics, another - better - sniper took charge and we were able to push Nod back....