I'd really like to have a way to to specify an alternative tuf url, if for nothing else then for root-signing (the TUF repository) testing.

• a flag like --instance <URL> would work but it does not have to be that
• the UX complication is that there kind of needs to be a way to provide the initial root.json as argument (alternatively we expect users to drop the file into correct place in local metadata store... but that is the tricky bit that I'd like to handle). So then you need to provide both <URL> and <INITIAL_ROOT> at least once

There is not a lot of detail here now -- I'm filing to not forget this again, will try to outline some solutions next week.