OK, I found the answer myself.
The property is available via app.GetProps ( ), which is what I expected in the first place. But the following code just gave an invalid object as result:
var MyProps = app.GetProps ( ) ;
var i = GetPropIndex ( MyProps, Constants.UserSettingsDir ) ;
var TheProp = MyProps [ i ] ;
It seems that the Constants.UserSettingsDir value is NOT defined in ExtendScript, or not correctly defined. Replacing it with the value 152 (which is listed in the Frame Scripting Guide) does give me access to the property:
var MyProps = app.GetProps ( ) ;
var i = GetPropIndex ( MyProps, 152 ) ;
var TheProp = MyProps [ i ] ;
Adobe, please get your documentation in order. It might also be useful to have a list of values translating to Constants.<Something>, so that we can at least track back from the numeric value in the Data Browser to a symbolic value to be used in the scripts.
Ciao
Jang