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.