You could use the bolt-on gutter-mount brackets (e.g., Yakima Sideloader or Toploader) on the side of the PH all the way at the rear and put a fixed bar there, then do tracks on top of the PH toward the front so you'd have flexible spacing for the front bar(s). That would allow you to get a bar almost at the back of the PH for loading your boat.
There are a lot of other options out there for making it easier to load a boat from the rear. Have a look at this page: http://www.prolineracks.com/kayak-canoe ... racks.html
In the past I've used something like the Yakima "BoatLoader" (http://www.prolineracks.com/yakima-4018-boatloader.html
), which extends out the front rack allowing for easy loading, assuming your vehicle isn't super-tall (might not work too well on a lifted SMB, for example).