I think magnets would feel horrible. I bought an expensive convertible jacket with zippers, from someone on TE (Bluetree?) at a reasonable price. The problem is, it's too heavy! It's really a winter weight jacket, because of the lining, so having the sleeves come off seems silly, because if it's cold enough to wear it, I don't want to be wearing a vest! The thing I love about this jacket, is that in addition to the detachable sleeves, it has pit zips. So, I can wear less layers underneath and use the pit zips, and not feel so warm.
Have you looked at PI jackets? My light wind jacket (the one with the funky zipper) has some reflective design on the back. It's not a lot, though. What about getting some of those LED blinky bands you can wear on your wrist and or legs? I have an old battery version that I use. Also, just my take on this, when I ride in the dark at 5:30 AM, with DH, his hi viz yellow jacket is clearly visible in the dark. No reflection needed. We have reflective stripes on our tights, though. But, people see our lights first,and they will not miss us.