I spent a great deal of my youth making free flight balsa models. I can assure you that an aircraft can be perfectly stable producing lift from the tail.
It's all about the moments baby!
I agree, but your basla models weren't carrying lots of cargo at varying CG positions! But I couldn't help but think it'd be hard to be sure that your tailplane was actually creating positive lift unless you had a windtunnel! I could dig up some resources on this and find a solid answer as to why they are generally designed to have the tailplane producing downwards lift but it essentially comes down to safety, utility and maneuverability.