Hi Noodle
to the Surface Engineering Forum.
I'm not sure if there are any significant bi-metallic corrosion issues between stainless steel and weathering steels. I know weathering steels basically rely on being uncoated and forming a stable rust/patina protective surface. Interference with the formation/ageing of a stable protective film can render it being no better than normal steel i.e. pointless choosing cor-ten if you plan to paint it.
I think a more important consideration will be how you plan to attach the stainless steel lettering. Avoid forming any crevices where moisture and dirt can build-up.
(sorry link now dead)