You can always start a new trial site. As long as you don't publish it there's no risk associated, you can keep pretty much as many trial sites active as you wish.
Creating a duplicate trial site then copying changes over is pretty cumbersome.
I was hoping more of an SVN / GIT type "preview" URL for clients to see before they sign off.
No reason you can not use a coding platform like GIT/SVN and have your revisions go in there. I do. Coda 2 for Mac does this really well.