Sure, although if you want to taste it I'd use more than 5% (I'd go 10-15%). Brown malt was the signature malt of porter brewing before black patent malt was patented. Brown malt adds a dry, roasty, coffee sort of thing that I really enjoy.
I just wasn't sure if that was apppropriate in a Robust vs. Brown porter as I never see it in Robust recipes. I'll up the % and give it a go.
On a side note, I was checking out your blog when I was considering doing a majority munich for the base. Good stuff. I think I'm gong to stick with MO, though.