I'm sure there are two threads on this forum about this. Unfortunately I don't have the links.
One used a plastic zip tie (cable tie, not sure what you guys call them over there) and zipped it to the dip tube, then once a few pints have been drawn from the keg you can stick your hand in and clip it off.
The other thread had something on the top of the lid to attach the ball chain to. Or maybe I saw something like that for sale somewhere. If you could find some sort of SS loop/eye or something I'm sure your local welder could tig it on for a bottle of beer or two, would just take a couple of seconds to tack it on.
Also a few years ago I tried making mint flavoured spirit from plain vodka, I left between 0.5 - 1 oz in a 40oz and after a couple of weeks it went a horrid brown colour and tasted absolutely ****-house, nothing like mint at all. I've never done anything with beer and (spear)mint but unless is a tried and true recipe I'd use a lot of caution.
Hope that helps a little.