Simple question: did you ever see a wag bag on the main trail back when there were toilets? Of course not.
No, if people weren't close to the solar toilet, they left their solid deposits in the grass in the bushes closest to the water crossings that they stopped at (and took water at) or at higher elevations, in the rocks near the trail. At night, sometimes even in the trail. During the busy season I normally started up the trail at 2am, about half the time there would be crap somewhere on the trail before sunrise. Sometimes they took their TP with them, sometimes not. The function of wag bags includes trying to change that. Does your solar toilet proposal continue to require wag bags elsewhere along the trail or not?
Are non-biodegradable items in a solar toilet a big deal? Not really, they fall into the basket and get hauled off. The National Park Service deals with it probably every day in the summer at one of the hundreds of backcountry toilet somewhere.
The National Park Service has restrictive camp siting rules to control where and how many (and not very many) people go and a fee structure charging for all entry. Are you suggesting those changes for Inyo NF as well, or are you making an unfair comparison? How many hundred solar toilets do you suggest the NPS "probably" operates in conditions similar to Whitney, economically, legally and environmentally?
Dale B. Dalrymple