I've considered adding a sparge process to my BIAB procedure. However, to me the key advantage of BIAB is everything happens in one pot. Once you add a second pot as an HLT, I think batch sparging in a cooler quickly becomes a better idea.
How are you going to heat your sparge water without a second pot?
I've done batch sparging using the bag to lauter with one pot:
1)Heat all the water in my boilermaker
2)Add the bag/grain to the cooler, open the spigot and fill the cooler up with my mash water.
3)After the mash I transfer the grain bag to the kettle which has the sparge water.
4)When I'm done sparging I remove the bag from kettle and dump the wort from the cooler into the kettle.
But again, to me, the big advantage of BIAB is the speed and ease of it. So once you start bringing coolers, or pulleys, or electric to the setup it looses that. I like being able to lift up the bag, plop it on the strainer, and begin the boil instantly.