Collection bins for used wag bags would be very similar to a toilet system, but without the privacy. This works well on on Rainier in conjunction with toilets. Note that the title of this thread is Solar Toilets vs CARRYING Wag Bags. People don't like being forced to carry their poop up and down the mountain. That's why many people choose to leave the bag of crap by the trail for everyone to stumble by. Many (but not all) pick it up on the way down. Collection bins would mostly solve that problem, but some will just go on the ground. Toilets generate the highest level of compliance and provide best environmental compliance, period. But there are people who feel strongly that everyone has to take full responsibility for their poop and the only way to do that is to carry it to the summit and back down to the trailhead. Sounds great, and lots of people do just that, but not everyone complies, hence the problem.
As stated throughout this thread, wag bags have their place in the toolset for backcountry human waste management. Even if the old toilets were replaced (Preferred Alternative 1 in the original Environmental Assessment) at Outpost Camp and Trail Camp, it would still be a good idea for people to use a wag bag on the upper mountain above Trail Camp or in between toilets if necessary. Use one if you wanted to camp away from the toilets. Same for camping on snow where you can't even dig down to the frozen ground. Wag bags have their place, but as predicted, they are not working well as a stand alone replacement for the backcountry toilets that were removed.
I'm due for another letter to Inyo management, and I'll again put forth the idea of a trial system at Outpost camp. But everyone should realize that this will only work if the people responsible for maintaining them are on board. Without Ranger buy-in, they will fail no matter how good of system is installed.
