Hello,
I have a new house built back in 2007 in Chino, California. I confirm there is a single dedicated 15 AMP circuit that serves all three bathrooms in the house. A single GFCI outlet is the master to 4 slaves.

Now I want to add a heated towel rail that is fixed on a walla and not free standing. I also do not want to run the long power cord to the existing GFCI controlled outlet next to my wash basin as it looks ugly. I rather install a new GFCI outlet to service the .5 AMP rated heated towel rack.

The problem is that to join the new GFCI outlet to the existing bathroom circuit, this would require me to do a lot of drywall and paint work.

Instead there is a common wall between the bathroom and master bedroom that has an existing 15 AMP outlet from a bedroom circuit that I can easily tap in to without much dry walling and paiting to do.

Now if I simply daisy chain a GFCI outlet to service the bathroom from that bedroom circuit... would that still be a condsidered a mortal sin?!

Your plan to extend the bedroom circuit to the heated towel bar is fine, as long as you do use a GFI device to protect it.

No mortal sin, not even venial involved, no Catholic guilt. Pull a wire through the wall, use an old work box and hook up the GFI. Commonly done when access is a problem. Make sure, however, that another 5 amps won't overload the BR circuit.