This.
If the co-ops exist, the meta-app is not a difficult app to build.
Geolocation to match you with the local collective and an API to send the booking request to them (unlike Uber they want to work with metasearch companies). Done. Let the collectives handle the certification, pricing and local routing.
(I know someone who founded a business effectively doing meta-search for a ride with local minicab companies via nationwide telephone enquiries, so it's not like the basic idea wasn't possible even with yesterday's technology).
Trouble is, for the most part these cooperatives don't exist. Which makes the meta-app less useful than Uber in most parts of the world. Uber has every incentive to discourage people from using a meta-app as their first port of call for ridesearch, and so even if their TOC were completely unenforceable they'd just find ways of blocking a meta-app from accessing Uber data. Blocking people and drivers who are obviously hacking Uber's service to co-locate each other, not paying for the ride via Uber but both ending up in the destination anyway is even easier. A non-profit meta-app isn't going to win an arms race with a multibillion dollar enterprise whose business depends on being a more reliable way of booking rides. Legal restrictions on reverse engineering their software isn't their moat.
The problem is that Austin is an edge case. Drivers got together and formed it and raised money to build the app and set up a vetting process the city govt approved because they'd lose their livelihood if it didn't exist. RideAustin doesn't exist in most other cities because drivers and passengers aren't really set up for collective action, particularly not against the company sending them rides, the costs of getting the service working aren't trivial and any nascent startup would have the problem that Uber was more recognised (and would likely try to kill their app stone dead by subsidising Uber rides with VC money). If you want more local cooperatives, you need to bring down the software development and vetting costs to run a service like RideAustin, not reverse engineer Uber's iPhone app. Or lobby more local governments to find reasons to bar Uber and Lyft from more city centres. And the market will see more cooperatives happen organically if/when Uber tries to squeeze more profit out of rides...