If you’ve already got a spare little block there wouldn’t be any harm in trying it out in the garden. It sounds interesting but one of those ideas that makes you think that if it was better than the conventional way it would BE the conventional way.
The title of the question implies that you’re looking to reduce friction, but I wonder if there’s really a problem there to solve? Might the strop tend to twist and cause the line to snag when most needed?
Presumably you would rig the reefing line both up and back down on the same side of the sail, which might (I’m not sure) allow the sail to bag out a bit more than usual on the blind side when you first start the reefing process; but this wouldn’t be a deal breaker.