I don't have any direct experience with hot bulb engines, so I can only repeat what I've read about them.
Most hot bulb engines are low compression, around 5:1; as a result, they don't have the high efficiency of a true diesel. That may not be a problem if your fuel is "free"*.
Hot bulbs tend to get cold when the engine is lightly loaded, to the point that they will quit if left to idle for a while. Running them under heavy load can cause the bulb to overheat. They work best at a constant load.
The hot bulb engine still requires some kind of injection, although they typically don't use as high an injection pressure as a diesel. They might tolerate poor fuels better, but would still be subject to corrosion (especially from acids in WMO).
Part of the problem with using waste oil is going to be carbon and ash. Both can kill an engine. If the carbon builds up mostly in the hot bulb, it might not be too difficult to clean out on occasion. But I suspect that the carbon would still get on rings and valves, requiring frequent cleaning. And the ash is going to go everywhere you don't want it to (rings, crankcase), and it's abrasive.
A big part of the equation is going to be if you get your fuel for free, and can tear down and fix the engine cheaply and often, then it might be economically feasible. But most hot bulbs are ancient machines with few spare parts available. I would think they would be expensive to repair.
Don't let me discourage you - if you can get an engine and afford to play with it, let us know how well it works. I suspect you'd be better off with an Indian Lister clone and plenty of spares. Lots of people around here have real experience with that, and can advise you on what works.
*-even "free" fuel costs money - collection, filtering, centrifuge, etc.
