ALindley: I was able to make it work on SSO enabled portal. There are 2 things that needs to be done in order to achieve this:
1. Set sling:sudo cookie with the user name that you want to impersonate as.
2. On Server side, using user admin or any service user that has priviledge on /home/users, set the rep:impersonator property as current user on the user that you are trying to impersonate as.