Currently I beleive there is no variable to enable this, as a workaround you could create a seed preference file with the designated required character and point to this using MS_USERPREFSEED.
This will be populated if a new Workspace User is created or the existing *.upf is removed from the clients install.
However setting MS_ALWAYS_CREATE_UPF = 1 would create a new upf file each time MicroStation is started.
The downside of this approach, is the user would loose any settings they have changed.