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