yes. you can do this failry easy. There are ansible playbooks and some readmes. There is some mailservice baked in, which you might have to change.

The only 'harder' part is (the last time I tried): You have to build the browser extensions you want to use, and override the path to the mitro server to point to your server. The config file is all centralized. So you change it once and can build all extensions at once. I did not find an options to customize the mitro server via the extension itself. (this is probably a good thing).

To get your customized extensions to your users, you might have to create an extra download site or fix the links in the mitro webpage.

You can actually change the server in a hidden preferences page. Go to /html/preferences.html in the extension.

that makes me wonder if it would not be an interesting idea to have a plugin that points to a known proxy and you register your host with it. then the operations cost is low, a single plugin can be released, and you control the server and data. just a random thought (typing on mobile with no spelling check, please excuse typos).

A password manager really needs to be a high-availability service -- it should work even (especially) when AWS is down. Since our service (intentionally) does not cache secret data on the client, running a proxy is not substantially easier than running our service. Plus we'd have to write this proxy :)