First thing I have to note is that this really makes me want to offer anything as a web app. The browser offers a much better experience in so many ways and security is a well thought out integrated experience unlike these 25 year old operating systems bolting security on as an after thought. Clearly no one at Apple cares about this but that would be funny if this was the crack in the dam that broke down their hardware software monopoly.
Second thing is why can't a third party offer this as a service? I'm not limited in the number of apps I can sign technically, right? Why would people using my app care that the certificate says it is signed by me instead of (trusted by the os) ABC, corp that (Microsoft|Apple) says in their overlaid dialog they trust. They could revoke something in the chain but it's technically possible right? Is this explicitly prohibited in some EULA I accepted in a brain fog?