Thanks for your reply.
No Tableau Online doesn't have that role - that's the problem I think.
Unlike Tableau Server, Tableau Online doesn't have the "System Admin" role and therefore impersonation is not possible.
The role with the highest privileges in Tableau Online seems to be "Site Administrator Creator" but that's what I am and I can't get impersonation to work.
You are right. After checking in here:
Highest site role for Tableau Online is Site Administrator Explorer
You probably needs to report this is Tableau Support and wait for their enhancement of rest API in the coming patch(es)