Butyl does need to be in compression to be effective.
A good idea would be to form a glass riser where the chainplate emerges through the deck - it will avoid water pooling around the chainplate. You can put a plate on if you wish. The link below explains - scroll down a bit.
Maintenance Log for Syzygy
Without a plate added to the design of the picture above do you think it will hold well. Seems like the space around the chain plate filled with butyl will get embedded with dirt.
Also without the plate where is the compression coming from?