White Daisy,
I've heard of this being done...several of my classmates who want to do FP have told me that FP's sometimes work or moonlight in small emergency rooms. From what I know, these opportunities are more available in rural or underserved areas, basically, in hospitals that have a hard time finding ER docs to hire. Certainly your options are somewhat limited in that respect, as I think most areas that are desirable to live in probably have no problem recruiting ER docs. But if your dream is to have a family practice clinic and work part-time in the local ER, all in a relatively underserved area, then you can probably do this. I've also heard of some FP's moonlighting on the late shift. If you really want to do emergency medicine, though, do the residency, as you'll be much more likely to find a job.
Geddy