The only option I can see would be if it is possible to archive a bulk of the old signatures as a pdf file that can be linked on the current petition page. Just have it say something along the lines of "this petition contains the 10,000 (or 5,000, etc) most recent signatures. To view the rest of them, please download this pdf file". Blah blah blah. You get my meaning. And the PDF can be updated every 5K signatures or so.
I wanted to add this on my last post. But the designation Jay put on this thread doesn't allow me to edit my previous posts. Sorry.