I developed a "suck start" system for recirculating through the bag on BIAB last year. The system worked very well, but the bag would tend clog up due to the circulation through it. It used a 12 volt pump, and copper pipe that went over the edge of the pot to the bottom where it was under a false bottom to keep the bag out of the suction line. The copper pipe was 1/2" and had an L on the end inside the pot. It went up to the upper edge, where there were two elbows and a very short piece joining them with vertical piece going down directly into the pump. A flexible hose returned the wort. Circulation was started by starting the pump, and sucking on the end of the hose briefly.... not even long enough to get a mouthful of wort / water, just enough to get wort down to the pump.... basically starting a siphon.
Sanitation is not an issue obviously during the mash, as you are boiling afterward, though this was one of the first criticisms absurd though it was.
The object was to be able to circulate past a thermocouple connected to a PID controller running an ordinary hot plate for positive mash control. It all worked quite well except for clogging the fabric in the bag making it more difficult to drain........(stuck mash).
I've been day dreaming about in bag circulation systems where the motor, perhaps just an electric mixer power head, would be above the mash tun driving an impeller inside a tube. The impeller would be a simple thing like a propeller, and it would push liquid down in the center of the bag, drawing from just below the surface. This would eliminate bag clogging as well as providing excellent circulation and a constantly moving stream of wort past the thermocouple. it would be used along with a hot plate and a PID controller.
That said, I no longer mash in a bag........ I use the bag only as a filter, pouring my mash into a bag in a colander. It works better for me. Your mileage may vary.
H.W.
