I think this is something that's going to depend a lot on the specific program that you go through.
Some medical physics programs are run through physics departments and in addition to the didactic medical physics coursework, the students have to complete mandatory graduate coursework, and pass the same comprehensive exam that all the other physics students have to take. Coming through one of these programs, you likely wouldn't have too much trouble qualifying to work as a physics lecturer (beyond general competition). I know of one person who taught a first year calculus course shortly after completing her PhD and another who took a position as a session instructor for a first year physics class while completing his PhD in medical physics.
Other medical physics programs are more professionally oriented and focus almost completely on the didactic coursework and then building up clinical experience for their students. In these cases getting in to teach physics would be more of a stretch, particularly when you would be competing against a pool of candidates with more academically oriented backgrounds.