Unfortunately this won't help a whole lot. The ULB is a very simple, low power piece of equipment. The key to the low power consumption is due to its simplicity. To incorporate a transpond functionality requires a detector to run on the ULB. So lets crunch some theoretical numbers.
Utter blonde moment on my part! good point, forgot any receiver would require a power source too and probably in excess of that used up by the ULB.