Actually, you can do this:

You can boot the machine; when it asks for an nvram password, you can pull the NVRAM chip out !carefully! and press enter; it should accept a null password, then you can pop the NVRAM back in and reset the password or change security levels.

Yes, this sounds scary! Yes, it worked for me. NO, I do no recommend that people go around randomly pulling chips out of their sockets, but I was stuck with a sparc 5 in total lockdown mode that I just had shipped to me, and there wasn't anything else do to...

good luck,

cc

-- 
Christopher Kyin-hwa Chen <cchen@nougat.org>
<http://www.nougat.org/~cchen/>
Time flies like an arrow.
Fruit flies like a banana.