I had a similar idea about making my ultralight self bailing by extending the seat into a full length inflatable floor and cutting bailing ports into the original floor (and reinforcing around them) since I found that the seat alone gave nearly enough buoyancy to keep my behind above water. I imagine that with a full length inflatable floor + a slightly thicker seat you would get self bailing capabilities without getting too wet (though being damp is a given)
Unfortunately finding comparable fabrics is proving difficult unless I buy on a B2B platform like alibaba. The lightest stuff I can get appears to be 210D based and is much too heavy for my liking.