Odd thought just went by:
There are sandpaper-based sharpening systems ("scary sharp" and it's cousins), generally using fine wet/dry sandpaper on a flat surface like plate glass.
There are motorized versions of this, typically spinning a disk of sandpaper mounted on a similar flat plate. And I know narrow-belt sanders are sometimes used by knife makers.
It just occurred to me to wonder whether an appropriately mounted/jigged sander I already have on hand -- belt or disk -- could be (ab)used for this purpose.
So: Outside of the usual problems of holding the blade reliably at appropriate angle and not overheating and the like, can anyone suggest reasons this wouldn't work? I suspect buying or building a proper slow-speed grindstone would be sufficiently better to justify its cost, but I could see this possibly being useful on a worksite....