I've seen this feature on a fair number of balleur BBKs:




Having machined a few sets of BBK brackets for my car, I think it's a nice idea. It puts the air right into the rotor hat. I have been fairly annoyed with duct backing plates rubbing tie rods and such, so I've already ditched them and just pointed bare hoses right at the caliper, so the "backside" routing works well. The dimensions of the oval should be fine for 3" duct to stretch over, which seems to be an okay practice judging by other duct kits.

I'm mostly making this to post up some monoblock milled prototypes, but I think it could be pretty easy to make the duct tube press in to make it simpler.