I can remember using a branson unit with flamable solvents, it had a nice stainless steel tub and was really well built = $$$
I would imagine that if one was to use the harbor freight units, you could fill a beaker or metal container (soup can or other even) with your flamable solvent and set that into regular tap water inside the cheapo ultrasonic cleaner.... Just to keep things safe.
And yes hot solvents work immensely better than at room temperature, in a past life of precision optics we used to use acetone at 150 degrees F to dissolve waxes and polishing pitch daily, aside from degrease the odd machine part... without much regard to flash point I might add, so be careful.