Is it feasible to use MV fluoroscopy to monitor the treatment port for left-sided breast cancer with DIBH?  

Absolutely.   When we first started doing DIBH, I used to stand at the machine with the MV fluoro on to watch the treatment.  It was indeed very reassuring.  As I recall, I was able to see the beating heart beneath the block, or maybe was just able to see the edge of the beating ...