My last practical idea was of course shot down in flames, so before i say anything more, i just want to stress that this assumes informed consent, is self-administered or part of a fantasy. Whatever floats your boat.
We all know about diet coke and mentos. The most important active ingredient in mentos from that perspective seems to be gum Arabic, which provides multiple nucleation sites for the release of dissolved carbon dioxide. However, if it's swallowed, it tends to lose those sites due to
So, this is my proposal. Take powdered Gum Arabic and calcium carbonate in a substance such as gelatin which is digested in the stomach (gelatin may not be the best choice - maybe the capsules for medicine would be better). Carbonate a dilute citric acid solution - many soft drinks are in fact just this plus a few other ingredients. Drink the liquid, then swallow the capsules.
When the capsules reach your stomach, they will gradually dissolve, releasing the powders. The Gum Arabic acts as nucleation sites for the carbon dioxide in the drink and the calcium carbonate combines with the citric acid to liberate more carbon dioxide, leading to inflation. All you have to do is hold it down, so the safety mechanism is simply to belch.
It would need to be carefully balanced in order to prevent a big shift in pH in the internal environment. The carbon dioxide wouldn't be absorbed to any great extent because of the relatively small surface area and the presence of the mucosa, and the Gum Arabic.
I'm pretty sure this is doable and i'm thinking of trying it on myself, but i haven't worked out the "yield". What do you think?
I think you'll run into more problems with the carbonated drink anyway, it'll lose most of it's carbonation when you swallow it.