As mentioned in my original post:
I have been able to work around this by specifying a ScriptsToProcess entry, which just did an Add-PSSnapin of the needed snap in.
So, if an
Add-PSSnapin WDeploySnapin3.0
is run on this machine it works fine.
The error messages output are consistent with doing an Import-Module rather than an Add-PSSnapin to make this resource available for use.
Don