I would suggest setting the FontString in XML to inherit from an ingame font initially, then change it from Lua when your config loads using
Fontstring:SetFont().
Alternatively, you can try your luck with
CreateFont() and inheriting it in your XML.
Lua Code:
local NorFont=CreateFont("EKPlates_NorFont");-- This is the global name we'll use when inheriting this font
NorFont:CopyFontObject("GameFontHightlightSmall");-- Initiate to an ingame font
NorFont:SetFont("Interface\\AddOns\\EKPlates\\media\\font.ttf",12,"OUTLINE");-- Set our custom font
XML Code:
<FontString parentKey="value" inherits="EKPlates_NorFont" justifyH="CENTER">
Note: Last time I tried creating fonts in Lua, there was a bug where the font couldn't be used even after using
Font:SetFont() unless I initially inherited from an ingame font first.