Hi Chris
On my 1937 14 the side tanks and bow tank are "sealed" by small fillets of wood spanning each set of ribs with drainage through small screw thread bronze caps. I attach a photo in which you can just make out the details. I say sealed but the reality on a boat of this age is that with the deterioration of the oiled membrane she leaks between the hull veneers and as a result water enters the tanks through the hull. I am happy to accept some leakage into the boat but less so into the buoyancy! The solution seems to be the same as yours viz to lift the tank tops and fill them with foam (or ping pong balls!).
By contrast I have taken a different approach on the Fairey (1957) and replaced the small drainage bungs with round access hatches just aft of the main thwart both to air the sealed tanks when ashore and allow access for inflatable bags inside (see photo). In this case I was more interested in making her bullet proof for practical use than in a strict restoration.
Robert
Int 14 K380
Int 14 K680 (for sale)