Allison resonded to my question about P/OS:
First DEC(Compaq) still holds the copyright and
it's free for use, not
free of copyright.
Yes, but apparently it's freely distributable. And in the absence of a
license, I think that implies that derived works are freely distributable
as well. My hypothetical reverse-engineerined "source code" would obviously
be a derived work.
Of course, IANAL.
Eric