Thanks. I installed Onboard but it isn't available on the screen locker or login screens under LM17 Cinnamon, despite setting the "Show when unlocking the screen" option. I believe the display manager used by Cinnamon, MDM, may not have options for offering accessibility features at login or screen locker screens (someone correct me if I am wrong).
I did find info on replacing MDM with LightDM (
https://sites.google.com/site/easylinuxtipsproject/2), which does offer Accessibilty features (including Onboard) at login. I tried that and was successful in being able to login with Onboard, but can't get past screen locker or switch user screens when they activate. I haven't been able to figure out how to use LightDM's screen locker instead of MDM's/Cinnamon's.
BTW: The LightDM info I found involved adding a Guest account under LightDM. Unrelated directly to access via virtual keyboard, one downside I experienced was that the Guest account began triggering Apparmor messages. It would be great to get access via MDM to avoid this potential downside, but any solution will be welcome.
Edited: to add the link with info on using LightDM