Thanks to a great description of a chainplate through-deck sealing method by Alan Berens and following discussions on the Valiant e-mail list, I have sealed the chainplate-to-deck interface on EGRET. Here's some pictures of how I did it:

Fiberglass tubes and 4-part mandrel. The mandrel was wrapped in plastic sheet, then wrapped with fiberglass cloth and mat in alternating layers and saturated with epoxy. When it set up, the bolts were removed and the center two pieces driven out, freeing the tube.

The chainplate slot was enlarged and the plywood "hogged" out. The lower fiberglass skin was not touched. The chainplate was re-installed and pencil marks made on the deck to help in centering the tube.

Ready to insert the tube.

A pencil laid flat on the deck was used to mark a cutting line just above the deck plane.

Here's the cut-off tube with alignment marks made on its top edge.

From the top.

Again.

The deck was masked-off. Duct tape was placed on the underside. Then about a 1/16" of un-thickened epoxy was poured in to attach the tube to the lower skin and seal it for subsequent pours. When this epoxy set up (2hr or so), epoxy thickened to catsup consistancy with coloidal silica was poured in. Since the deck slopes, the cavity was not completely filled. After this pour jelled, the hole was topped off with a thicker layer of mayonaise consistancy epoxy.

Here, it's all filled, sanded and the lower skin has been cut back to the size of the inside of the tube. (Actually, it's a different chainplate hole; I forgot to bring the camera to continue the story on the next day.)

Here, we're ready to pack in the butyl rubber. Actually, one side and the ends should be packed, then the chainplate slid in and bolted in place. This will smush the butyl rubber on one side and the ends. Then fill the other side, half from the top and half from underneath.

UPDATE: It turns out that the butyl rubber was really tough to work with. It sticks to the chainplate and doesn't want to be pushed down into the gap. We re-did the staysail chainplate and used a caulking compound that we got at a Home Depot. It was much easier to apply, as it sort of flowed down into the void. It's called:

  Henry Elasto Caulk #289
  "White elastomeric Acrylic Patching Compound/Roof Coating"