Given the above, I would think a better solution would be electric
radiant
heat, which could be easily installed exactly where needed. *Given
it's only
*supplemental heat for one room, the operating cost shouldn't be very
high.
You;d have a control, timer, etc in the bathroom.-

yeah my suggested solution too, easy to install, just run when needed.

if the OP were putting in a new ceramic floor they could imbeed
electric floor heat grid, but its probably not worth the effort.

or a combo fan heater for the cieling. they really warm things up