Signal doesn't support messaging without any number, no. You can interact with it programmatically though if you give it a dummy number (even twilio, etc work fine). I personally have a REST endpoint running on a server that has its own number just to be able to get notifications and so forth from my server when I need it.

I wrote the software with the intention of allowing it to be used as a Twilio-like service, but I'm not sure how much utility anybody else would get from it. The messages from the source to the API obviously aren't protected, so the only use case it has is convenience rather than security. The lack of a signal implementation in a sane language (I'm interacting with signal-cli, which is a wrapper around the Java one) makes this a lot more difficult to just drop into other random tools unfortunately. I might just end up releasing that service as an open source tool if other people find it as something they'd want to be using for their own purposes.

Signal also has some pretty heavy rate limiting on things like numbers which are annoying to hit because things just tend to break. They don't have any other way of preventing spam and crawling of the service though, so I completely understand it.

As far as I know, you need a phone number. I think they mean that if you install Signal, it'll invalidate Haven's key.

Right. Haven has its own registration, if you register the number again it won't be able to use its own keys anymore.