Yes
Leisurepro sells somthing like that:
Scuba Accessories from LeisurePro.com
This sort of thing could also be nice for those that need to save space and get cylinders to fit inside a shell w/o need for making extra holes for the valves (like on IDA-71s).
My concern is loosing the ability to easily change the 1stage if something is not OK. Or how easy it is to reach those tiny valve knobs.
Anyone knows if these can be had with M25 threads?