Unless Twitter angers their users greatly there's not much that can be done. Twitter stands for status updates just like Google stands for search. A competing product would have to have a (slightly?) different feature-set I think..

Is it fully compatible with status.net already?

Edit: I tried to post from my status.net client choqok and it failed with "Creating the new post failed. The result data could not be parsed.". Is the api url https://rstat.us/api ?

It's meant to work, for example see here https://github.com/hotsh/rstat.us/issues/387

The client api and the backend that implements the protocol are different. I mean, you simply need html to provide a means of building a client. But, no client will really do that. rstat.us will try to support existing apis, but that's not there just yet. The backend in rstat.us does support ostatus, and can interoperate with status.net, but does not have the same api as of yet.