Hi Larry,
As you mention, VM:Backup 3.6 has Signal Shutdown support, it was added during the development of VM:Backup 3.6. At that time Signal Shutdown support was also added to the VM:Manager Common Nucleus (level 5.1) which is the current development version.
For the products that currently run on the VM:Manager Common Nucleus (VM:Tape, VM:Schedule, VM:Secure/Director, VM:Archiver, VM:Operator, Explore), the Common Nuclues support can be retrofit via a PTF for each product.
Research will need to be done for the remaining products (VM:Spool, VM:Batch) to see what can be done to add Signal Shutdown support.