NB ABAP can work as a service provider (consumer of IdP) not as an IdP which NW Java can do. There is a significant difference.
NW Java, can however be configured to use ABAP user store, which you then could consume in the ways you mentioned above.
Generally it has been my experience that when companies start thinking about SSO they rise above just thinking about SAP now-a-days, so haven't seen this implemented nor know of any specific doco.
Cheers,
Chris