Can the user defined facts be used to support GEDCOM defined facts that are not currently supported in FO?
Surprising to me, No! The user defined facts all become one of type EVEN which is the supported event tag. You cannot use this feature to define mapping from foreign programs which I had originally hoped to do. Foreign GEDCOM files with level 1 tags that are unknown to FO will be ignored on input. You could edit the GEDCOM file and modify it to map these facts to the supported general event structure but it is a lot of work.