Hi,
This is definiteily possible, but not OOTB. You will have to write some servlets and UI elements to have AEM act as an IDP. Looking at an implementation I have done for this in the past (which, sadly, cannot be open sourced), it is somewhere around 2000 lines of code. The actual implementation was more like 4000 lines of code, but that also included LinkedIn integration (some of which is now available in ACS AEM Commons).
Regards,
Justin