The SAR modes will use other sensors (GPS, accelerometers, inertial nav, even doppler) to give the speed which is why they are allowed to control TU/TD at low speed.
If the aircraft had a low speed display (LVI or groundspeed on the PFD/ND) then it could be used by the pilot for low speed IF but whether such 'extras' would be specified during the certification process I don't know.
Exactly - which kind of makes the VMini a strange restriction if you have the other kit.
139s all display groundspeed, and provide a "hover display" as an HSI option, to show low speed vector (both direction and speed)