Yeah it's called car cancer really, no matter the make.

I've seen some self-repairs that can be done even if it's perforated, but they are temporary & hard to make look nice. Especially if there are sharp body lines.

 

Now that HondaPartsNow is making s2k parts. I did find a full rear quarter panel, that would then need to be installed, primed & painted. I'll have to get some estimates & see if this is the most cost effective thing to do.

https://www.hondapartsnow.com/genuin...s2a-310zz.html

 

Spray the wheel arches from the inside with your preefered brand of rust proofer or wax. Do it again after a few years.
The flat spot of the arch, where the screw of the bumper is located, need extra care. Spray it from the underside (Wheel side ) with your prefered underbody wax. Especially arund the plastic nut for the bumper screw.
The black trim pice around the arch is a good idea by Honda, preventig paint chiping from gravel and water from creepin between the panels; however, it need to be removed from time to time and controlled. Again, use your favourite brand of rust proofer before you put the black trim back on. Something greasy is preferd at this spot, marine grease for example.

 

Did you remove the rubber trim from the rear arches yet? Because if you didn't you're probably going to find out that it's worse than you think.

 

The panels have always been available from Honda. Don't know where you heard the rear quarters aren't replaceable. They are, just like any other car. It's a hell of a job. Lots of spot welds to drill out. I doubt you could get it done for less than 3k per side unless you have a friend in the business. And it's virtually impossible to seal the joints like the factory does. Many of these cars that have already had a rear quarter replaced develop new rust at the joint in the door sill.