Can motor run capacitors be used in high frequency applications?
You get what you pay for. Using 60Hz caps on a 3kHz (example) supply will shorten the life of the insulation by roughly 50x (ie 100k hour design gets reduced to 2k hours at same rated voltage). Not only that but most line frequency caps are cheap paper/foil/oil. Low loss at 60Hz but that quickly gets lossy at kHz frequencies if voltage isn't reduced. You might get away with it for low duty cycles and/or voltages but if it heats up enough you can turn the metal can into an unintentional pipe bomb. Then there's the parasitic problems you mentioned. Personally I'd just spend the extra cash for proper caps (sounds like you need metallized pp film ones) rather than risk it.