Was looking in search for this but didn't find it and not sure if it's going to work
It is just a thought....
To do this u will need a warlock and a rogue. So rogue goes to enemy base and hides and warlock is summoning him and as soon as he get accept pop-up he takes flag and click on accept button. And he will be next to warlock who is in a base.
I dont know if u can summon in BG's but if u can this is a nice way to win And if u want fell free to flame :P