The reason there are a bunch of threads about temp gauges on automatics is because they're far more sensitive to heat than a manual transmission is. The #1 killer of automatics is heat. That's also why there are external coolers for automatics as well. Automatics also have fittings for the express purpose of running lines to a cooler whereas a manual won't have the provisions to do it.
There isn't really much of a need for a transmission temp gauge on a manual transmission unless it's a high performance unit like would be used in NASCAR or F1 racing, but if you really want to add one you might be able to remove the bolt that's aft of the fill plug and use that for a gauge probe. If that doesn't work, you might end up having to drill and tap your own hole.
