Ive R&R'ed more than a few bladders from aircraft. The bladder top surface is all that's held up, small SS clips bonded to the top surface attach to hangers inside the upper wing/tank surface. They dont hold up the whole weight of the tank and fuel, just the fabric tank skin. Just to keep it from collapsing.
All tanks need to be vented. If you opened up the ballast tank to install a bladder, the bladder tank area should be a totally isolated compartment, completely sealed off from the ballast tank. If your concerned with not having quite enough ballast with empty fuel tanks (18 gallons fuel x 6 lb per gal =), just epoxy 100 pounds or so of lead shot into the bottom of the ballast.
All outboards have a fuel pump capable of drawing fuel from a tank at a lower level, so that shouldn't be a problem.
However. I am not sure that aviation tank bladders are capable of tolerating ethanol fuels, and they are very expensive. What you might be better to do, is open the ballast tank, and build a compartment to carry a standard metal 18 gallon fuel tank. You could glass in support structure, then seal it off so as to retain your ballast tank, and leave an access panel to the fuel tank. Then you could have fuel gauge, etc.. Plus, it would comply with EPA (as well as your insurance), should something unfortunate occur.