Of course you can. That won't be Signal though, just an another network using its code, and there's already XMPP, so there's no need for that.

> (and thus doesn't allow other apps to connect to their servers)

"thus"? You can easily use centralized model and allow the client ecosystem to thrive. You don't have to do much, just don't actively prohibit them.

And I know the blog post, it isn't very convincing.