Edwin Pawlowski wrote:

Access panel is a good idea. Often they are in the room on the other
side of a bathroom if it is an interior wall. I've never seen one
that you can buy, but rather they are built in when the plumbing and
drywall is done. Just a wood frame and a sheet of 1/4" hardboard or
plywood screwed in place.
I had a similar problem with my shower and had to make a hold on the
partition wall next to the vanity. Rather than patch the drywall, I
hung a mirror.

Hanging a mirror under the toilet would certainly generate questions...