Not enough demand to pay the bills.
I can envision a design for a hot hide glue gun. As John has noted, it couldn't work like a normal glue gun. After the basic development costs we'd need a guaranteed 1500-2000 customers willing to buy a gun at $125-150 each (wholesale) or $250 retail to make any profit. Or maybe 10K customers at a lower price.
Like most extruded stuff, the trick with purfling is that there aren't enough guitars being built with the stuff to even justify the cost of setting up the machinery. I've got at least 50,000 guitars worth (if not 2-3X that) of custom-extruded black side dot material in a roll in my shop...selling the stuff would probably be more hassle than what I'd get for it. If you know someone, though...
I'd be really keen to see the books at StewMac or LMII on sales of specific items, to see the sort of market there is for very luthier-specific tools and materials. Kevin's done alright with the zip-flex, but it's a high markup item with relatively simple manufacture and he did most of the work for his own use before he started selling it. I wonder how many benders Brad Way would need to sell (if he decides to) to offset what the engineering costs would have been (had he been paying himself for his time)?