Allopathic residencies are not allowed to fill their positions except through the match--unless that program specifically has positions meant to be filled outside the match (meaning those positions cannot be advertised/filled though the match--it's either one or the other). To my understanding no PM&R program does this...
If I'm not mistaken, DOs may be allowed to pre-match. But if the program is not allowed to offer a position outside the match, it makes that point moot
Edit: At least I think that's how it is... I could be wrong--I know some programs have offered positions outside the match, and they were banned from the NRMP for a few years. Which is a pretty big kick to any residency program. But I think programs may still be able to set aside positions to fill outside the match, but this is seen more in less competitive specialties at undesirable programs. (While PM&R may not be the most competitive in terms of board scores, it is quite competitive considering the number of applicants and the number of available positions--no program has gone unfilled through the match for at least a few years now.)