If I were you, I would NEVER upgrade just for this, it would never fix your problem and it simply sounds like whoever you spoke to in TAC used that as an excuse not to do any work to help you further. Im literally positive that even if you had R65, issue would be exactly the same. Yes, you can use route based vpn, but I do know that in R80.xx versions, there is an option in guidbedit to actually turn off supernatting for specific tunnel, though that might not really fix your issue, but worth a shot.
Andy
Best,
Andy
"Have a great day and if its not, change it"