Seat pan
The old Valley seat in Avocet RM was plastic pan,almost hull hugging on the inside, and foam insert.
It looks as if plastic pan is attached to the hull using two screws, not a whole lot modification required for your approach.
I like your idea a lot.
Update
Well, since I couldn’t find anyone to help me out with judging trim, I tried the field expedient approach - ethafoam and duct tape.
So, here is what I did:
I attached 2inch block to the back band and had to move footpegs by two notches. That sort of makes sense since the yakima braces are notched at inch separation.
Here is what I observed:
The paddle gets loaded much sooner, that is maximum resistance arrives quicker than before.
Boat became more responsive to maneuvering strokes, no surprise here
Still weathercocks, but is, maybe, easier to control than before. Of course, the wind was just 11kt, so this could be subjective.
Catches waves much, much easier. Again we are talking about wind chop, less than 1foot. Stern still gets kicked around, but not as bad as before.
Sprinting gets funny - the bow starts bobbing up and down. Yea, man 
I think I’ll try to modify my setup to increase ease of adjustment.
The foam block needs some time to get used to, but works OK.
Also, I need to follow up on Patrick’s suggestion to use GPS to track changes.
One more update
Friend measured placement of the seat in his Avocet RM - 1inch closer to the bow than in my copy.
I’ll definitely have to try that