To answer your question - no.  I know of many admins who have transferred to different departments.  One admin at a previous company who supported the president was the current facilities manager.  (It should be noted the company was bought out and her president resigned.)
I agree health first.
Second before going to HR I would block out time very shortly after coming back off sick leave (without duties at home) to discuss the needs of the current president.  It sounds like you need an assistant or he needs more than one assistant.  Before giving up the ship are there any responsibilites that can be off loaded to another admin?  For example if there is a sales and marketing admin can s/he be responsible for scheduling meetings and you handle the agendas and handouts?  Can any of your functions be automated?
Third: I would also set very definite time limits on the use of my time.  If I can not work over time but am expected to answer his calls at 9:00 at night - something has to give.
Fourth and last: I would also learn how to push back.  Make the boss set the priorities. When a new assignment comes along. let hime know what tasks you are working on approximate time to complete and would he set the priorities.
Good luck and let us know what happens.