Do DIN rail mount interface relay modules with a "spare" terminal exist?
See Phoenix Contact FBST, it's a bar that goes in the slots under the wire connector you see on the blue relay module.
Finder will have similar. 093.16 for example. See their catalog.
Keep an eye on the total current running in this bar.

Tip: when looking at this kind of industrial products. Find the catalog PDF! It has more seachable info than the websites. Eg: eaton, phoenix, finder. Anyone with this much products.