Waterproofing (rather, water-resistance-ing) will be fairly easy to handle. I have easy access to 3D-printing devices and will just design an enclosing with a short space to place an o-ring around the edges.
Your point about small+light is however valid and the reason why I discarded solutions based on a arduino+daughter board. Power supply: I was thinking of a 1S LiPo element, anything above 1000 mAh should do the trick for, say, a full day of operation, maybe two. That would maybe go up to a weight of 25-30g, max ?