The train station in Inverness definately has left luggage facilities.. but I think you can only leave stuff for a maximum of 24 hours..
I you are staying somewhere in Inverness for a night either side? I'm sure they would keep it for you..
Otherwise pack light.. I have had the same dilemma on occasion, and have an unltra light pair of trousers, really thin t-shirt and some crocs that must only weigh a couple of hndred grams in total, so I dont mind carrying them so much.. The crocs are handy for around camp and rivers anyway