I am PM&R PGY4 who applied last year. I am sure you know, but it is going to be an uphill battle. I met few people who were in practice, but they seemed to have more complete package than I had and still struggled to get interviews.
If you went to PM&R program with some academic weight, have few pain-related research publications, and a letter from pain guy, then it might be worth applying. In other words, you must have a bit more complete application than current PGY3s to have a decent shot at matching. If you think you are lacking, I would go for 'lower-tiered' or PM&R run programs. You should also apply for PM&R Spine programs as they generally are easier and more acceptable to red/yellow flags than ACGME programs.
I would say PM&R PD letter is a must for every program. Many program's website information is outdated or lacking complete information.
Regarding programs accepting PM&R candidates, I would say it changes every year and more and more programs at least try to be 'multidisciplinary.' I got interviews from what is known as 'non-PM&R friendly' programs in the past. If it is in the region where you want to practice, then just go for it.
I know it is a bit harsh, but you are welcome to think otherwise. Just my 2 cents.