By doing what you describe, you're tying down an engineer (that could be spending that time planting/sweeping mines) and a tank, that could be bleeding your opponent, for a potential +60% repair speed bonus at Vet2.
Not to mention whatever unit is damaging the tank. It's good to see people try and think how something might get exploited, but the cost/benefit ratio to do so in this case seems ludicrously high. Vipper's suggesting of tying it to the minesweeper upgrade has potential merit as well.
I was just recently wondering if this mechanic could be implemented, so I'm interested to see it play out.
As for Ost heavy tanks call-in reduction being tied to T3, that seems like a very minor improvement over the current situation. We might see some T3 units, but if you have a Tiger commander then T4 will probably continue to be forgotten. That's only speaking academically though. I'd have to play to see for sure.