So your 30 minutes ago post grabbed my attention, and as I was re-reading the thread from the beginning, I saw your comment from a year ago:
And wanted to ask: did you ever try that?
I remember thinking that might be good until I saw the "Toasty, Biscuit, and bold, sourdough flavors", when I decided against trying it.
Never did the faux Maris thing. I used that Special Roast in a porter and a pale ale a while back. I think it comprised 3-4% in each of those. I don't recall it adding much to either beer.