What options do you have? Can you run a new return line for a circulator pump without a lot of work? I personally would not send hot water back down the cold water line. I don't know about other people but I like my cold water to be cold. Also you may find that your shower is now too hot until cold water flushes all the hot water out of the line. Can you run a new circuit? If so a tankless on demand heater would be your best option. If the heater can't heat up cold water as you use it you are going to have a hot shower that turns cold before to gets hot again as the slug of cold water between the storage tank and the POU heater makes it's way up to the bathroom. Since it's an upstairs bathroom I would think that your options are limited.