500 ma at 5 VDC may be pushing the normal view of what a transformerless power supply can run at but it is by no means undoable or any less unsafe than any other transformerless design, just a little larger. If the circuits are all encased in plastic and non of the control circuits requires any external connections other than through buttons or switches there is no reason it cant be run off a transformerless power supply system.
As far as finding AC line rated capacitors in the correct size I would suggest looking at motor run or small power factor correction type capacitors. They are designed specifically for working directly across AC line power and work quite well in larger transformerless power supply applications. Also being mass produced they are rather cheap as well.