It seems IAM was the copyrightholder (or just licensee with the ruins of C= still the copyrightholder?) and stopped selling it in 1999:
http://web.archive.org/web/19990203071727/http://www.iam.com/amiga/The domain iam.com was sold soon after, judging from archive.org.
It appears Heinz Wröbel kept working on Envoy til v3.x, selling through Schatzruhe.
Your best bet would be to try to dig up Heinz Wröbel as he seems to have been the last person to work on it, thus likely the current copyrightholder. Maybe he's willing to put it in the public domain.
Couldn't find a Heinz Wröbel after a quick google search. Only found a dead German newsreader of almost the same name.
