While I haven't done a DPT program and reserves at the same time, I have been both active duty army and a guardsman and will be starting DPT school fall 2015. I think it would be possible to do both and I definitely understand the allure of having tuition assistance and other financial benefits through the reserves. You could arrange to make up things like annual training, guard drills, etc, but I think it would be quite a strain. Instead of having breaks from school you will be spending your time doing something stressful. I could see anyone getting burned out fairly quickly. You also have to think about things like military schools, deployments, NTC rotations, etc.
In short, I think it is do-able, but I wouldn't want to do it.
Another thing to note is that most units will require you to do make ups beforehand. As far as clinical rotations go, I had a soldier that completed an internship for school so he came in during the summer, made up his drills, and I didn't see him again for about five months. If you are really serious about this I would seek out the training NCO at the unit you plan to join (as it is probably local) and run this past him or her.
Also, if you are not already in the reserves you are going to have to find time to complete basic training and whatever initial training is required for your job. In the Army the minimum length of training is about 14 weeks plus probably a week or two of hold. If you want to be an officer then count on more initial training and time commitment once you get back to your unit.
If you are already in the reserves and think you can handle it I would say go for it, but I would caution you against joining, signing a lengthy commitment, and then starting school.
