@flybigjet nothing wrong with that setup, just an extra step in the beginning. You can tie a piece of bungee cord into the same loop as a suicide strap and get the same result for basically no money. It's a common setup in the UK for 7' hoses
You and your suicide strap. It's a bungee necklace! This isn't the rebreather subforum!
Like tbone said, I made the hose retention bands for my sidemount tanks using bungee cord. But, I tied them using a double overhand knot, instead of two Fisherman's knots, like you'd do for a bungee necklace. The double overhand gives me something big to grab when I have gloves on, to pull the elastic away from the tank and stow a hose.
The only potential problem that comes to mind immediately in doing what you are talking about for single tank back mount is will the hose retention bands get cut or otherwise damaged by being pinched between the tank and whatever back plate/BCD tank plate you're mounting the tank on? Maybe you can put the hose retention straps at the top and bottom of the tank, above and below the tank straps and above and below whatever plate or rails the tank is being secured to.
Maybe you could just make small loops of bungee that go around the tank straps (assuming you have 2). They would sit naturally at 90 degree angles to how the long hose would run, but it's bungee so it can twist and stretch and should work okay.
The other problem would be, how do you re-stow the long hose if you pull it out during a dive? I wonder how the BSAC guys handle that. I guess you could just Hog loop it (around your neck) for the rest of the dive and re-stow it after you got out.