Hi,
I'm new too, so if there are people with more experience I'll be happy to be corrected, but...
As I understand it you're allowed any sort of tea made just from leaves (no fruit teas or flower teas). Obviously you can buy ready-made tea bags, but I can't believe an infusion would be substantially different and not allowed. I've been making my own by ripping up a few fresh mint leaves (as I have some in the garden) and pouring boiling water straight over them, and I seem to be doing fine!
A friend recommends Nettle and Peppermint Tea (she buys them as tea bags) so I'm considering trying to make some of that from fresh nettles as well (I understand that the stings disappear when the leaves are washed).
I haven't done it yet, but I keep thinking I'll make up a big pan of mint tea and let it go cold - think once it's chilled it should make a "legal" flavoured water...
Again, haven't tried it, but I don't see why you couldn't use that sort of water to mix with the chocolate shake and end up with a mint-chocolate shake, or even a mint flavoured Hot Chocolate.
As long as you only use the flavoured water and don't eat the leaves themselves, I think anything goes...
Good luck!