The thing that I failed to make obvious is perhaps the most important reason for wanting a plenum like this - you get to ditch the horrid elbow! Custom TB mounts are not a problem either, we just make the holes bigger

The other thing I've designed in is to make the other endplate extra thick, just like the TB one. This means that if you so choose you could mount your vacuum takeoffs and sensors there so that the front plate is nice and clear if you're putting the extra injectors there. You could also use the spare bosses that are already there to mount the second fuel rail.