What you describe sounds lovely to me! I'd hate to paint it.

While a silver or reflective paint might buy a little mileage - I'm certainly not sure how much.....I was planning to experiment by putting a mirror in back of an east facing zuke plant - but the plant didn't take off at all so never bothered...

yes it is an awful pity to paint the wall as it is an old stone wall that is quite attractive.
Also I wonder if I would be cooling down the wall to the same extent as I was reflecting the light and so would be loosing out on that source of heat.

Instead of painting the wall could you put white painted removable panels in front of the rock. of course then you would loose the heating of the rock. As I use rock to heat up and moderate temperatures in my my pepper garden I question the loss of this heat. If you do paint use white, the gardening under lights forum people have info on the reflectivity of white and aluminum paint. I am on the 45 parallel.

Hey lucy - since your focus is light and the wall is doing it's job heat wise - perhaps some solar panels that would power during the sunlight and then give some light afterwards would accomplish your goal?

I would also love to see a pic of this - which is practically a pipe dream of mine to ever have....

Even if I wanted to build one - it wouldn't look right as the house and garage is a sided frame structure....

But would love to see yours!