Tim - I did your modified mounting method on my former canoe.  I mounted it with a screw through the ash thwart.  I didn't like it.  Even with moderate stress it would still rotate and I never could get it tight enough without damaging the holder or the wood.  In hindsight it was a dumb idea but maybe your set-up will be more stable? 

I don't use a rod holder anymore.  I just tuck the butt under my leg and pinch it behind my knee w/ the rod extending across the canoe in front of my other knee and resting on the opposite gunnel.