It's probably obvious to experienced VirtuaWin users, but FWIW on a suggestion from mouser, I was able to successfully have a dock stay visible on all of my VirtuaWin desktops. This was achieved by using the 'Always Show' option for the dock in question via the control menu [1] for the dock window (I needed to select 'Manage Window' via the control menu first before the 'Always Show' option would appear). Perhaps this will help someone some day...wait, may be that will be me when, inevitably, I forget how to do this
[1] Note, the control menu is a VirtuaWin feature.