(st d) I think burping noisily is more offensive (in a polite setting) than farting. You can always do it silently and the "product" is inoffensive. No-one need know. But you can't get away with a fart, however discreetly expelled. (Projoy) Now, there's a challenge I can't ignore, as a chemist. The simplest answer would be an appropriately-shaped pellet of activated charcoal taped to the bum in a gastight manner. It would need to be kept dry and be easily removable for obvious reasons. It would eventually (a month or so) become "full", so to speak, but could easily be regenerated by heating in an oven, the only problem being that everything it had absorbed would be driven off. Extractor on full blast, I feel. snorgle has got in while I was writing this and the link says it's been done already. I bet it's activated charcoal, though. Just a question of how you fix it.
[snorgle] There's a satisfied customer quote on that site: "The feeling of security!" from one Julie Anderson, which I initially read as Julie Andrews. Now there's an image.
The whole problem with that product is the need to make the underwear airtight. Ewies. I did read once upon a time about tablets you can take (which I believe are also charcoal) to nullify smells. Any health risk?
[snorgs] Thanks - it's a fantastic site with a wealth of snigger potential. 'Wear them for the one you love' springs to mind as a suitable phrase for examination, given that possibly the most irritating and in-yer-face flatulence occurs beneath the duvet ...