Does
THIS not fit on a new MSR dromedary bag?
I have this. My DIY gravity filter system uses Kent Systems Acrolene tubing and Kent collection 4 quick fittings, so I have a very short length of tubing and a quick fitting attached to the cap which allows me to very quickly connect it to my filter.
When I look it up (MSR Universal Bottle Adaptor and Cleanside Cover), I'm finding
THIS which appears to be the same but with a slightly new look. In any case,
THIS looks like it would achieve the same goal as well.
When I'm using my system, I just swap back to the other cap when the drom bag is full, but you could easily just keep the "tubed" cap on and install a valve in the tube that you can open and close. Or an even lower tech option, at least with Kent Systems fittings, is to have a cap fitting that you can just plug that short tube with, but I'd carry an extra or two just in case one got lost since it's a pretty small piece.
Kent Systems (Acrolene tubing and quick couplings) EDIT: Re-reading your situation, it sounds like you're trying to get a quick-connect that matches the hydration pack style one of the Hydra. I guess you'd just need the one half of
THIS to complete your equation.