I got the boat out and careened, and the downhaul HAD found its way alongside the board, jamming between the board and the trunk. I suspect this didn't happen with the original weird attachment point because the cable always had a straight shot from its attachment point to where it exited the trunk. I suspect the new attachment point directed the cable at a sharper angle towards the "v" it had sawn in the top of the trunk, causing way more friction, too.
I ended up dremeling a new hole in the forward top of the trunk, a bit above and aft of the stock uphaul hole. I made enough room in the new hole for my stock block to relocate there, then sealed the two layers of the hull together to prevent bilge leaks and misery. I used polyester resin. 5200 seems like a better choice but I didn't have any on hand. Hopefully this works. It's easy to access and do better if necessary.

Installed the stock block, routing the downhaul over the block and horizontally forward, then re-rigged the stock line and pulleys between there and the mast. I was torn about whether to rig them so you pull back (easier to pull, but required putting a new hole in the cuddy wall, like the later boats) or forward (more awkward, but allows an easy rig with a bungee as a fail-safe). I went with the latter, though it will be easy to change in the future as it's all just clamps for now.

In this image, you can see the block where the cable comes up on the far left, then there's a clamp (wrapped in tape) and a thimble connected to the stock lines and pulley. There's a bowline in the end of the line, so I tug it forward til the board's down, loop it around the mast and then connect it back to itself with that little bungee with the blue ball on it.
Went sailing today and it worked great--only thing I didn't foresee is that it's a little annoying to wrap the downhaul cable around the mast and pull it back aft because I keep my anchor bungee'd to the base of the mast. I was able to tuck the line through between the two and I don't think I'll change that rig--in the water with gravity helping, the board went down and back up with incredible ease. I haven't addressed the original hole in the trunk yet... My original thought was to grind it open enough to have a 1/2" threaded fitting with a cap, but on reflection that's removing an awful lot of material. I think I may clean it up prettier, fill the holes, and modify a rubber stopper to fit the oval hole. Easier, cheaper, less cutting the boat up. I don't know how in the world I would have gotten the board out without being able to poke through that hole, though, so I'm definitely NOT going to seal it up completely.
I also solved my (unrelated) motor issues. My efforts to solve the bilge leak have only been mostly successful, though, so I'll have to pull the boat off the mooring ball and haul the thing to the yard AGAIN. I fear it's rudder gudgeons.
Thanks for all the help. I'll post an update here if there's any more relevant details.
Luke