I would imagine it's a very individual decision for any program in any specialty. However, if you were wondering who qualifies to get an out-of-match offer by NRMP rules, then here are the categories:
Student/graduate of an osteopathic medical school
U.S. citizen student/graduate of an international medical school
Student/graduate of a Canadian medical school
Non-U.S. citizen student/graduate of an international medical school * (including permanent residents)
Student/graduate of a Fifth Pathway Program
Previous graduate of a U.S. allopathic medical school
If you are one of the above, then a program may offer you such a position. I might be wrong, but I have never heard of a program setting specific requirements for an out-of-match placement.
Also, if you have completed a year of EM residency anywhere, on the SAEM website they show openings at various programs with their requirements (haven't seen one that didn't want a year of EM experience tho)
Speaking of SAEM website (Residency Vacancy Service). There was an opening in Philly and they were considering candidates with prior non-EM training. "The Emergency Medicine Residency at the Hospital of the University of Pennsylvania is seeking an applicant from either an existing Emergency Medicine Residency Program or physicians with prior non-EM training, to become part of our PGY1 class."
But it was posted quite a while ago, so I would be surprised if they didn't fill the spot yet.
The other way this happens is if a program increases the number of residents in their program. Sometimes the approval does not come until after the match. In this case, there might be open spots in a PGY-1 class.
The match is about the only real chance of getting an EM residency PGYI position.
Emergency medicine offered eighty-four more positions in 2010 compared with 2009 (1,556 vs. 1,472), representing an increase of 5.4%. Eighty-one more positions (5.3%) were filled in emergency medicine compared with 2009 (1,540 vs. 1,459), with thirty-six more U.S. seniors (3%) selecting emergency medicine (1,182 vs. 1,146). So the scramble was for 16 positions in 2010 versus 13 position in 2009.
1 in one hundred of total available positions are up in the scramble is not very good odds. Most likely almost all scramble positions going to seniors of US medical schools.
SAEM residency vacancy service occasionally has a PGY1 Position about two to three times per year.
Another source is find a resident. Which in the last four years has posted only one open EM PGY1 position. Unfortunately, There is a $75 subscription fee for applicants; active ERAS users pay a discounted $30 fee, but you can see for free, if there are any openings, you will need to pay to get the name of the program.