Now This would be Cool (I think :-))

Andy, I've just looked on Aliexpress and you can buy refillable Co2 canisters off there. They look like they have a threaded join half way up. You could buy a few of those and see if Mike can turn up up a longer cartridge to extend it

have never been a fan of hanging large heavy co2 bottles off co2 guns,and dont like eposed co2 lines either just to fussy i guess bout weight and looks eh,have made up quite a few custom made bulk co3 bottles for different guns,allways done in such a way it didnt destroy the oridional gun,so cuold be put back to origional simply be removing bulk fill bottle,had made several refillable recycled 88gram co2 bottles out of empty ones,i bore the necks out /thread them and fit two way valves in i make myself for guns that use these bottles this a good way to keep origional looks but get the cost advantage of cheaper bulk fill etc,its not hard to make special bulk co2 bottles for most co2 guns evan the pistols that use the little 12gran cylanders can be bulk filled with specially made small co2 cylanders if you got the time and equipment to doso,nothing is impossible but miricals do take little longer.

Yes, this looks like a possible alternative, however as Dave metioned it's a bit bulky.

I do like the idea of a 88grm, I could do put those on my cr600 to make it lighter for stalking with.