Manual transmissions are NOT necessarily towable. More often than not, the lube is distributed by the rotating cluster gear which is powered by the input shaft. When the engine is not running, the input shaft is stationary and no lube is distributed. The center mainshaft needle bearings and output shaft ball bearings will eventually run dry and self-destruct. Be sure you know how the transmission is lubricated before setting the vehicle up for towing four down!
SOME older automatic transmissions may have a rear pump driven by the output shaft. That pump MIGHT provide lubrication to the bearings. Such vehicles often can be push started by getting it up to 35 MPH in Neutral, then putting the transmission in drive. The rear pump will supply sufficient hydraulic pressure to engage the clutches or bands.
A driveshaft disconnect may be the best bet for towing four down. It is likely that a good fabrication shop will have to custom design and install the baseplates to match the towbar.
A trailer to load the vehicle might be an option, depending on the likes/desires of the operator