I've just run across the password clearing procedure in the VAXstation 3100 
Model 76 Maintenance Guide (EK-VS31M-MG.002) 

The steps are:

1. Power off.

2. Remove the socketed Dallas chip. The manual has a diagram that shows
where it is 
   but I'll have to settle for a textual description. It is a socketed chip
with a white picture
   of an alarm on it. It lies to the back of the two sets of memory sockets,
roughly in the 
   middle (but behind) them. If you have a graphics board installed, it
looks like you need
   to remove it to access the chip. The chip is keyed, so make a note of the
orientation!

3. Sit the chip on the conductive surface of the PSU such that the leads
make contact.
    
    DO THIS ONLY FOR 1 TO 3 SECONDS. (The manual has a big bold warning - do

    this for longer than 3 seconds and you may damage the chip's internal
battery).

    This shorts the internal battery briefly and resets the password (and
presumably
    everything else ... if it matters note it down ... except since the SHOW
commands
    probably don't work that might be a tad hard in your case!)

4. Put the chip back in properly.

5. Power up and breath a sigh of relief (if it worked ...)

Antonio