I am running True64/OSF1 and FreeBSD/Alpha on several boxes.
From my experience, I guess it is not difficult to compile
clisp on these plathomes.  I had to modify sources at several
places, though.
I could compile gcl on True64 5 and OSF1 4f. OSF/1 binaries can
 run on FreeBSD 4.5/Alpha.
I compiled clisp with Compaq's ccc on linuxemulator of FreeBSD,
and it worked perfectly.

Good luck!

yoriaki fujimori