In the bath, we put up one of those double-tier towel racks and, of
course, it's 30" long which means I can only anchor one end in a
stud...the other end had to go into drywall. Rather than use those
tiny plastic expansion anchors that come with the rack (beyond
useless), I used a molly anchor.

A few weeks ago I noticed what looked like the wall puckering outward
around the end of the towel rack with the anchors (not the screws into
the stud). Sure enough, the drywall in a 2" oval is puckered/bowed out
and the wall is soft/flexible there.

It looks like I may have hit an old repair, or the horizontal seam of
the drywall, as the top of the oval is actually straight, like a seam.

Obviously Plan Z would be to cut the bad section of wall out, patch,
repair, re-paint. But we JUST painted the bathroom, and I'm loathe to
have to do that section of wall again, given the long list of other
projects I have to tackle before our niece moves in next month.

So my question: any ideas on how to give a solid mounting surface to
this one end of the towel rack that's got bad drywall? Some trick to
insert a piece of 1x3 or such (for screwing the rack's bracket into)
inside the drywall without creating too large a hole to cover?