Wow - 40 views and no response. Must be a common issue. Well, here's what I did tonight and it worked beautifully. Long ago, I bought some of the small hand clamps from Home Depot. These are the plated steel with soft orange plastic grips and tips. They are very strong and when you squeeze them to open them, they probably open about an inch or so. I slipped the orange plastic pieces off of the clamping ends to reveal the thin, flat, plate like steel jaws. I was able to insert the bare steel jaws into the spring about a coil back from each end. When I released my grip, the clamp did a reasonable job compressing the spring - enough to be close to the size it needed to be to install.
I then placed one end of the spring over the head of one of the studs in the mirror and pushed on the opposite jaw to further compress the spring and work it into position. It worked remarkably well. Once the spring was in position, I removed the clamp, reinstalled the plastic end pieces and threw it back in the tool box! I then had to work the spring a little to get it fully aligned on the pins but that was no problem. I was thrilled and I think you will be too.
Incidentally, I used this same type of clamp (leaving the plastic on the clamp) on the window glass when I was reinstalling it. Positioning the clamp at the glass and door location prevented the glass from being able to fall down in the door.
Hope this helps others,
Harry