I think this would be impossible to code. I imagine Andy has set pathing for the creeps to follow. Having the player in game change the pathing would be impossible. Then again, I'm not a coder, so ^^
For example, in Bloons Tower Defence 5 the Monkey Apperentice tower has the ability "Whirlwind", that throws bloons closer to the starting point, away from the exit. Making this portal thing could work as when the start portal attacks, it has a chance to send the creep back to the end portal. Can use the same method as the Withered set uses to send creeps back in time. Or it could just be one portal that has the ability to send them back in time, just like the withered set. The chance should be more like 1% (+0.1% / level), because if it sends like every tenth creep back, it would be too often and it would break the game.
Blockland purpose was for empty/water blocks, not on pathways I like your idea for 1% (+0.1% / level) I am planning putting end portal to the last tower and the first to the first tower :3
Oh, I misread. In that case, yeah, sounds very much like the Withered set. I thought you meant change entirely where the creeps spawned and exited!