72hr free transit when holding an L visa

I currently have a double entry tourist L type; I've used one entry but not the second one. It expires in May 2015. I wish to fly through Shanghai in December 2014 (Hong Kong > Shanghai > London) staying less than 72 hrs.

Can I still get the 72 hr free transit, and save the second entry on my double entry L type for a future trip, in early 2015? I suppose I would have to be careful to tell any immigration officer what to stamp?