Hello David,
first of all I think RACI was extended by S in the standard but you are right this is not shown in tab RACI.
I guess (!) this can be changed in the XML configuration (in earlier posts it was mentioned that you have to buy a course and the related material to do it). What each user of published content can do is to use in the fact sheet "process" (this comes if you view BPMNs) not the tab RACI but the tab "Table". There you can add columns for many (all?) attributes including RACI attributes:
But the huge shortage is (as far as I know, @SAG please correct me) that you cannot save this configuration.
Regards
Klemens