There is a dinghy towing forum for this topic.
I don't know about the 1990s, but when I was looking in the first part of the 2000's, the flat towable pickups with 5-speed manual transmissions were Ford Ranger/Mazda B-series using the Mazda source M5OD transmission, and Nissan Frontier. Any other 5-speeds were questionable with respect to lubrication of the output shaft bearing, as that shaft was above the level of transmission fluid. In normal driving, everything was lubricated by splash from the gears on the input shaft. On the Mazda M5OD, Ford wanted the engine run every 500 miles to relube the dry parts of the transmission.
I've seen some unofficial recommendations for other transmissions about overfilling for lubrication, but I've seen nothing about the transmissions GM was using in that era. I know folks who have flat towed supposedly "untowable" Toyota 5-speed manual transmissions, but the tow distances after hookup have been on the order of 30 to 100 miles, so everything probably still has a good oil film before the end of the tow.
The risk of towing an "untowable" manual transmission are mostly economic. It is not like an automatic that is likely to overheat and "explode" or catch on fire, it is more like the transmission will have a shortened lifespan, and your risk is cost of repair, or replacement with another transmission from the junkyard.
