It'll be interesting right enough. The rocky mountain brass has a true .410 ID and takes a 209 primer. In theory you could use standard plastic wads etc. but as you point out the charge needs holding back momentarily, so without a crimp that holding back will have to be enabled through the friction of internal components and the shot it's self. Unless of course one employs a crimping die. I've a pal thats offered to make me a die but he reckons a tight wad and hotter primer set over the relatively slow 410 compatible shot shell power will have much the same effect. We'll see.
I don't have any data for brass cases as yet but yeah, I am looking into it.