James,
Looks interesting - a bit like a Razor and Nomad hybrid!?
I've been using a similar set-up with the Razor harness and a modified hydration bag as a BC, mounted vertically under the harness. The big problem I have is a lack of confidence in the hydration bag when it comes to leaks or punctures. I'd like to have another option for the BC, although I'm looking for something designed more with warm water and aluminium tanks in mind.
There's a few of us using the Razor set-up had the same heavy feet problem, most have solved this by lowering the BC or mounting it horizontally just above the hips - this is a lot easier when using the modified hydration pack and bungees but it's hard to get the BC under the harness when doing this! Using a set of recreational type plastic (or small rubber) fins helps too, it doesn't look great but it does feel much better for your trim.
Have you thought about adding additional D-rings on the belt to keep the ends of the tanks lower? I guess you're mainly be diving steel tanks so have less positive buoyancy issues on the tanks but I still think the lower profile from the Razor belt D-rings works better.
Karl