Thanks all for the suggestions on possible solutions to the towel rack
dilemma.

To provide feedback on some of the ideas:

Towel rings - solution of last resort. Wife really likes the double-
tiered towel rack because I like the bath sheet (BIG towel) being 6'
5" and all that and there's not enough room on a simple rack or rings.

Toggle bolts - not a solution; the wall is already bowed/damaged in a
diameter larger than any toggle bolt, meaning the bolt's just going to
exacerbate the problem.

Mounting plate - actually am considering it, and have a router to
finish the edges off nicely; only downside is the double-rack will now
be an additional 3/4" inch further off the wall in a fairly compact
bathroom, and did I mention I'm a good sized man?

Recessed mounting plate - might be the most favorable solution, as
long as I can figure out how to finish the seam between the plate and
the drywall

Solution I'm also pondering (whilst drying off from my shower this
morning) is a variation on what /aem/ talks about with a horizontal
blocking...cutting a small bit of the damaged wall out - small enough
the rack mount would conceal - and seeing if I can slip a mini-plate
behind the wall, something maybe 1/2" thick, an inch wide and long
enough to distribute the weight of the rack and towels over a greater
area of the wall.

Will let y'all know what happens...