Saw this on the blog and like the look of it. Particularly interested for my 29er.
I don't have a frame bag for it and wondered if the Gorilla cage would work on the bottle cage mounts to occupy the central triangle space?
Seems like a good compromise on the expense of a new frame bag.

Stu, do you think it would interfere with pedalling?

I guess it'll partly come down to the geometry of your frame / size of the main triangle. I've run the monkii cage in the frame on both a 29er (Salsa Fargo) and 26er (Soma Groove) with no problems of pedalling interaction. I'm hoping that the Gorilla will also be ok, assuming I don't overfill it (tall + narrow is the plan). Hopefully it'll come in the V-cage version to allow it to go in / come out at non-straight angles.

Stu, do you think it would interfere with pedalling?

It'll all depend how you pack it. If you're using 2 mounts (rather than 3 as you will be inside the triangle) you have the option to mount it high or low.