Indeed, one of the drawbacks of the completely smooth and uncluttered hull interior on the dory is that there is nothing to tie anything to!  A traditionally built boat would have gaps under every frame you could lace a small line through.

I did two things to provide tie-downs on my dory.   One was holes drilled through the frames.   The lower the better -- I put mine about 1 1/2" above the thwarts.  The holes don't need to be 1/2";  I started with 1/2" on the first frame I drilled, then realized that that hole looked huge and a 3/8" or 5/16" was plenty big for reeving small line through.

Holes through the frames below the thwarts will be inaccessible if you plan to put the foam flotation under the seats as specified in the manual.  

The second thing was wooden padeyes glued into the bottom and the side of the boat.  Tying to the frames won't hold things secure and in place low in the boat, it will just prevent them from floating (or sinking) away if you capsize or drop them over the side.   So put some padeyes at the level where the garboard meets the floor, on the garboard so you don't end up stepping on them.  You can use those to strap or tie things tightly into the boat, low enough to be stable ballast and flotation.  Note that one or two points will not be enough to strap a drybag in securely and make it immobile... that will take either two points and a 1" wide strap, or 4 points. 

 Buzzard, I cut two holes in each of my dory's frames through which I run  dowels with removable end-caps to hold the floatation in place (ie the dowels runs through the fore and aft pieces of floatation with the frame in the middle).  This allows me to easily remove the floatation for easy cleaning of the hull (I like a completely empty hull for cleaning) and other purposes like rowing.  I also have not screwed the seats in place, so they can be popped out for the same purposes.  Works well, no problems.  

   Thanks, good ideas..