"What do you mean, I can't pay in Meeples?"
Just FYI there's armor specifically for the back in the new version, so the torso armor definitely doesn't cover this area.
v2.1.? Or is there a more recent update?
The back slot is for Legendary Cloaks only up to v2.1. (as far as I know) and they don't have an innate DEF rating. Personally I've been using the Torso DEF rating for the Back as well because it doesn't make sense to me that you'd be running around with no armour there at all. The idea of equipping a shield there is tempting too... hm, decisions, decisions.