The OSD instability I encountered after the kernel update persisted though with less frequency.  I've finally determined that cause is a confluence of small issues that amplify each other:

• Power Supply Aging - The 5V 10A supplies have lost a small amount of their output headroom with age.
• CPU Governor Changes - The ondemand CPU governor is no longer as aggressive at reducing the CPU frequency
• CPU Aging - The CPU and PCIe controller appear to have become more prone to core undervoltage.

I've remediated the instability by underclocking the CPU.  Underclocking was insufficient on its own, so I've also applied slight overvoltage as well.  The OSD RPi's have been holding steady after applying both changes.

Here's the current state pf my usercfg.txt:

# Place "config.txt" changes (dtparam, dtoverlay, disable_overscan, etc.) in
# this file. Please refer to the README file for a description of the various
# configuration files on the boot partition.

max_usb_current=1
over_voltage=2
arm_freq=500 

These changes have not appeared to impact my Ceph performance.