In 4.9, Font Materials were added: However, they seem completely undocumented.
Is it possible to make a font outline (or other effects) through those?
During 4.8, sds-michael had offered a workaround which included editing the engine, has that been merged in?
(Unreal Engine 4.8 Slate font outline hack - SlateTextRun.cpp - Pastebin.com)