I am mot sure if you can can true TTL-BL when in the Commander/Master mode. But you could simulate it.
The Remotes do not have a TTL-BL mode available.
Your camera would need to be in the matrix or center weighted metering mode to get the TTL-BL to be active.
I see not mention of TTL-BL when a unit is in a Master or Remote mode, so it might not be possible. Remember for the TTL-BL to be computed the rear lighting of the subject from behind is measured and the pre-flash is used to compute the fill flash requirement. In the Master mode there is no pre-flash of the master while the remotes are pre-flashing for exposure. The pre-flash of the master is before the exposure flash of the remotes. This would mean there is no way for the Master to compute the needed fill flash if any remote unit were behind the subject.
As to the contacts. RadioPopper does not directly connect to any Nikon or Canon Speedlight. RadioPoppers use an induction coil to detect the flash charges on the Master and use an IR led to transmit the signal to the remotes. PW with their line of MiniTT1 and FlexTT5 do have all the contacts that the Nikon camera has. I see no mention of TTL-BL with the FlexTT5 so PW might not have programmed this into their product.