It'd be nice if SECURITY DEFINER functions could see what user invoked them, but current_user is the DEFINER user, naturally, since that's how this is done in fmgr_security_definer().
I was thinking that fmgr_security_definer() could keep a global pointer to a linked list (with automatic nodes) of the save_userid values. Then we could have a SQL function for accessing these, something like pg_current_user(level int) returning text, where level 0 is current_user, level 1 is "the previous current_user in the stack", and so on, returning null when level is beyond the top-level.
This seems like a simple, small, easy patch, and since I [think I] need it I suspect others probably do as well.