Softimage saves your preferences to %XSI_USERHOME%\Data\Preferences\default.xsipref when you exit Softimage.
If you want to save your current preferences, you can use the Preferences.SaveChanges() method.
Just drag this JScript to a toolbar to create a script button.
// JScript Preferences.SaveChanges();
For example, you could create a button on the Custom tab of the main shelf (View > Optional Panels > Main Shelf):
You probably want to make the button a Script Command (external file), so you can assign a keyboard shortcut to it.
eX-SI 
nice to see list issues followed up and resolved or worked around so often.
thanks for the effort Stephen.
Can the script write to a specific directory (C:\…”) as well as the “USERHOME/Data/…..” ?
No, in this case the idea is to save the changed preferences to the actual preference file.
You can export your preferences elsewhere
– in the Preferences dialog box, right-click Preferences and click Export
OR
– write a script that uses Preferences.Export()