It's definitely all about the volume.
Most who seek elec. heat are just trying to knock the cold out, and they generally aren't flowing more than .5 or .75 gpm, and they try to fill with warm water in the first place, so I do think you could make a 220v heater work, only ones I know of are the Volcano heaters, Fast Eddie has one he seemed to think it was adequate for his porty.
I think the trick to making the whole porty in the house thing work out on a two story 2500 sq. ft. house is 2" hose and parallel vac motors for 200 cfm, quite a few guys running 75' of hose with this set up, but it would drive me crazy, I'd be squirreling away some cash for a TM.