Even if you could make Olm/OMEMO interoperate otherwise, the Signal protocol supporting client requires access to a server to pick up pre-keys. So you need to h

Even if you could make Olm/OMEMO interoperate otherwise, the Signal protocol supporting client requires access to a server to pick up pre-keys. So you need to have access to the pre-key server of your correspondent or your correspondent needs access to the pre-key server you are on. The Signal Protocol is not generally suitable for this sort of thing mostly due to its complexity. If you did something stateless like OpenPGP then you could do E2EE over bridges with no problem. The cost would be that each and every message would be complete in itself. So sending a single emoji would take several hundred bytes. That might be less important these days...