Just an update here. I've had the balloons in my Heritage H575C for about a month now. One original one is still good; the other I deliberately punctured with a safety pin, to see how easy it would be to get it out. Very easy. Poke a small hole with the pin, & the balloon won't pop. You can squeeze the air out, & pull the balloon out with ease. These aren't "pro" balloons; just from a bag bought at a dollar store.
Really works as far as giving you more headroom volume-wise from your carved-top axe, with higher howl threshold, yet the sound is no different. I deem it a success! Better-looking than tape. Less intrusive than foam. Cheaper than those f-hole plugs.