As I read the first posts, I had a similar idea to kiwipom (your credit, I'm not stealing your thunder), using a 'grub-screw' from a LPG fitting. There is a variation I recently discovered that has a replaceable rubber seal tip, held in place via a screw in plug with ~5mm bore,and ball and spring behind (I expect for using two+ bottles to a system- irrelevant).
It's a bit chunky but maybe enough material to slice a different thread and trim to length so to not block the bush oil feed and use whichever spring most useful....or whatever??
Come to think any Allen-head grub screw could be drilled through but steel ones maybe a less satisfactory seating medium, or could maybe even make your own from Ally bar(drill and press in a hex before threading o/s)?
It needs to be simple and easy to service. I don't think locktight necessary, as 'where's it gonna go to?'
Hope that makes sense, it took a while! cheers, duTch