Tooky - take a look at the humanure handbook (downloadable pdf).
You should be able to make it work for you. We have a portable composting loo setup that we use when we have large gatherings or are spending a few days in the woods.
I recommend that you try a bucket system like he uses - when you visit for a few days just empty any deposits to your
compost heap and leave it clean for next time. Sawdust is pretty much the best carbon cover you can get for fast hot composting - absorbs all odours and liquids and composts hot enough to kill all pathogens.
Mike