Feature: Way to force a 'last item'

I'm trying to use linkme as part of writing extensions for other languages, such as PHP. It's working very well for this case, with one exception: currently, I have to clone the slice to add a null item on the end for lists that must be null-terminated on the PHP end. It would be nice if there was a way to provide a guaranteed 'last item' when defining the list, or something similar.