Hi JM and welcome to the forums!
I haven't forgotten about you... I was glancing over the MIBs to see what I could tell you.
Have a look at the s5ChasComTable MIB entry in s5cha125.mib and the OIDs below it.
If you don't have the latest SNMP MIBS for the Ethernet Switch (formerly BayStack) 450 I've uploaded them right here;
mibs_bs450_430.zipYou could also probably determine how many switches are in the stack by walking the interfaces.
Good Luck!