Blacknet,
Did I read your post right? Are you really advocating a pony bottle system that would require you to remove your bc in order to free yourself from entanglement or to hand off your pony to a OOA diver?
Please reread ID and omar's above posts and reconsider your choice. Now I am by no means even close to the most experienced diver on this board (a few here have been diving longer than I've been alive) but even I can see many flaws in such a system. What if you are entangled in such a way that taking off your bc isn't option, what then? What if your metal to metal connection becomes jammed, how do you remove your pony then? Removal of your bcd while underwater is not something most divers look forward to in the best conditions not to mention in some type of emergency situation. Personally I'd want several options before trying to remove critical equipment like a bcd. Additionally, where do you stow the 2nd stage and can you restow it as needed? Can you reach your pony's 1st stage to turn the pony on and off? How does your system allow you to deal with a pony freeflow?
As I said on another thread, I think the best way to carry a pony is rigged as a DIR stage. This has the additonal advantage that this rigging will carry through to stage bottle, something a metal to metal (ie Tiger Gear, I think that is the name of the company). As well as being much cheaper.
Also a stage/deco bottle is not the same thing as a pony bottle and unless you use "DIR type" rigging you will have to use a different type of rigging for stages.
How do you know you will never use mitts or dry gloves? I dive mostly in Texas and it is not that uncommon to see peole diving dry, especially in the late fall, winter, and early spring. Furthermore, how robust a system are you looking at if it can be rendered useless by the wrong style or thickness of gloves?
Pony bottles are intended to add an additional measure of safety to one's diving and to certain degree they do when used right. However more complication and failure points do not make for safer diving. Task overloading is a prime reason for diving accidents, don't make your system any more complicated than absolutely neccessary.
Dive safe,
svs3
P.S. As to your original question, I'm not sure there is any pony bottle "tank band" system that is as simple, robust, reliable, and cheap as DIR stage bottle rigging for carrying a pony bottle.