I can see that this is a difficult topic.
I decided to use a not so sophisticated solution by modifying the Ace libraries directly:
AceGUIWidget-Heading.lua
Line 28: self.label:SetFont(My_Font, 13);
AceGUIWidget-CheckBox.lua
Line 193: self.text:SetFont(My_Font, 13);
AceConfigDialog-3.0.lua
Line 512: _G[tooltip:GetName().."TextLeft1"]:SetFont(My_Font, 13);
Line 519: _G[tooltip:GetName().."TextLeft2"]:SetFont(My_Font, 13);
