As of last summer, the only true externships around that accepted students from other schools were RIC and Cinci (to the best of my knowledge). I had some communication w/ the Indiana program, but I got the feeling that its more of an informal program and they seem to usually take Indiana students.
To any students looking for PM&R experience b/w M1-M2:
In addition to the above 2 programs, I would highly rec. contacting a local PM&R residency to see if it would be possible to get involved w/ a research project and/or some shadowing in several different areas of PM&R. I really stress the importance of several different areas, as most students don't realize how broad the opportunities are in the field.