First of all thanks for all your comments, links etc. Now, back to the science of toupology!
We Know from Star Trek producer Bob Justman that several toups went missing over the course of Star Trek (not just Bill Shatner's , but Nichelle Nichols' wig too). Now we aren't here to accuse anyone - besides, the star of a show was, in our view, perfectly entitled to take a gift or two from a production. In this case, that would allegedly appear to be several of Bill Shatner's Captain Kirk studio-supplied toupees worth several hundred dollars.
So did a point come when Bill Shatner started wearing his studio toups outside the lot? They were clearly better than ones he might have been able to come by otherwise. The picture above is taken before/after the Star Trek second season wrap party. Unlike Nimoy, who would naturally have his pointed ears peeled off before going home, it seems that the Shats gradually began to prefer keeping the rug on outside the set.
The picture at the far top of this piece depicts Shatner doing one of his favorite things - motorbiking. It seems that at least for the cameras, he took the time to toup-up. Perhaps at this point, once the cameras were gone, he ripped the damn thing off his head before riding off into the desert.
Incidentally, the 1974 movie Pray for the Wildcats co-starring William Shatner is partly about motorcycling. It comes in the midst of Shatner's "Toupee Dark Ages" - the period between the cancellation of Star Trek (1969) and the buildup to the first movie (circa 1976), where a lack of money both personal and in the budgets of the films he starred in led to some very cheap-looking pieces on Bill Shatner's head.