Hey Matty,
I'll be interested to hear how you get on with the OAG, as I might try one myself at some point. I also had a lot of trouble with flexure, but recently made a bit of a breakthrough.
Having securely mounted on scope on top of the other I was still getting quite a bit of flex until I realised it was being caused by play in the scope focusers. I was corresponding with a guy in Sweden on another forum that had the same problem - I tried propping up the focus tube on my guide scope and he took it a set further by clamping his with another scope ring (
http://www.istarion.net/Picturepages/Autoguiding.htm). I've since done this to mine and the difference is HUGE.
I've still got a bit of play in the focuser of my FLT-110, which is a lot worse when I add a 2x PowerMate, but when just the camera is in place I'm getting only 0.2-0.3 pixel movement for a 5 minute shot
.
Let us know how you go.