Interesting... It is an intriguing invention. Why did no one think of that before?
(knocks hand on head...)
Yeah, there are lots of things. Most maternity clothing shops will sell a variation of some sort or another. People do everything from using a rubber band (looping 1 end thru the hole and then just hooking it around the button) to boob tubes pushed down to cover you undone jeans.
When I was pregnant, I cut the waistband off a pair of pants and put a wide cotton/lycra waistband on it.
What country are you in? I might be able to find some websites which sell various forms of a pants extender type thing.
When my wife was pregnant, she just got out her old jeans from before the diet, and cut the buttons off, re-sewing them on further apart. saved a fortune, and was able to enjoy outfits from before, which matched!
Of course, a zimmer frame is another possibility, with a little shelf on which to rest the tummy...
Can't really see it catching on, but at least it's somewhere to hang the nipple pads and maternity briefs...