I've struggled with this for a while and can't find any articles or videos showing how best to mask the soundhole for finishing. The Taylor factory tour youtube video shows that they use tape somehow, but it doesn't look well-sealed. Others suggest to blow a balloon in the soundhole, but I find that if you're a applying a wipe-on finish (e.g. TruOil), the proud bump of the balloon gets in the way.
Here's a photo I borrowed from the AGF that shows how John S. Kinnard masks his soundhole. Does anyone know how he does this? I've sent him an email through his site but haven't heard back yet. I can see a crease in the center of the mask, so he must fold it in half, insert it, then apply it somehow.
I tried a balloon for a brace jack once, Ron, and it didn't work. I couldn't get it to apply enough pressure so I had to rig another way to clamp it up.
I've thought about an inner tube but I haven't found one small enough... although, now that I'm thinking about it, plumbers use inflatable rubber plugs to pressure test gas and water lines. There may be some of these small enough to work.
I'm TOO familiar with the inflatable bladders that plumbers use.
Keeping in mind that air (gas) can be compressed and liquids (water) can't, it will take some serious PSI of air to get it to inflate. If it fails: catastrophic damage to the instrument is probable.
They have an expansion range of about 3/4"-1" max. (according to my bro-in-law who is a union plumber).
At our local Farm Supply store, they sell all kinds of supply line tubing. They have several spools of what looks like inner tube rubber in various diameters. Some creative gluing & sealing & stitching of one end could yield some results. It's sold by the foot so experimenting is on the cheap.
Hope that helps :) Have a good one :)