Oh correction 2012, this is the oldest WoB I can find on the subject, and says basically the most he has ever said about it

FireArcadia Is there any use to being a copper Compounder, from a Feruchemical point of view? I think the same point would also apply to an aluminum Compounder.

Brandon Sanderson Some combinations, like some abilities themselves, aren’t really that useful. That said, being able to Compound copper…that could do some things. Aluminum, not so much.

https://wob.coppermind.net/events/105/#e1131

I believe a Feruchemist would be able to use other sources of investiture, since that’s how compounding works, using allomancy as a fuel for feruchemy. And if you look up copper compounding, you will see that Brandon has been RAFOing that since 2015, but he has said it doesn’t do nothing (as opposed to some other types of compounding like aluminum). It’s possible that we might get a look into it during era 3, but I imagine that a copper feruchemist would have similar results feeding from a Perpendicularity as they would compounding. Whatever those results may be.