[artemist]

Yes, it is possible, if your attacker isn't too powerful. However, there will be a lot of tradeoffs and the software is not quite finished yet.

The software which is currently closest to this is ricochet [0] which communicates directly between Tor hidden services. Since there is no central server, there is no one to record metadata (at least in a trivial fashion). However there is still a lot of work left to do before ricochet is something that could be mainstream. Also, some metadata can be leaked, such as when one is online, and (until proposal 224 is out) how much people are communicating based on the number of lookups on the hidden service directories corresponding to the service (since there is no randomness in the system yet, afaik, you can brute force values to get into the right place in the ring. I haven't read the Tor specifications in a while tho, so I may be wrong)

You could also use some sort of mixnet system, although you would probably end up leaking who is sending messages to their ISP. To stop the ISP from determining when, you could send o constant stream of encrypted traffic, although that would be inefficient. (I need to read more research on this)

However, weather people will actually use these systems is in entirely different problem.

[0] https://ricochet.im