I'm not a lawyer but I play one in comedy clubs. The first implementation of 9p came about long before Plan 9 had a free (as in rms) license. Nobody got sued, nobody died, although a few bystanders were maimed.
My advice as your lawyer [in comedy] would be to go nuts and do whatever you want. The documentation[1] is a good place to start if you don't want to look at any source (no license required to see that!), and if you want to cover all corner cases, a running Plan 9 kernel is a good client/server to test against. ---- 1: http://plan9.bell-labs.com/sys/man/5/INDEX.html
