Cycled water is useless- there's next to no bacteria in water. Cycled gravel or filter media is what you need.
Tubs work swell- I use a small plastic filing box as an outdoor pond in the summer. Just don't fill it up all the way, or it can split down the side.
There is a solution to the ammonia levels, IF the room will be bright enough...
Only have 1 guppy per tank (In my opinion, I'd go with endler's livebearer only due to their smaller size. Their care is identical to guppies, and you might be able to go up to three if the bowls are at least 1 gallon. Also, fast the fish for 1-2 days (no food whatsoever) before the reception)
On to the solution.. If there is ample light (or if you're willing to invest in a small LED light fixture held over each one.. Google 'LED Puck light', and find one battery powered), you could go with a centerpiece plant in each tank. Not anything like those Betta vases- you can't cover the water surface, but something small could easily take up all ammonia. The best thing that comes to mind is small 'lucky bamboo'..
Pick a size large enough to stand out of the water at least two inches. In each bowl, add gravel from a cycled tank to the bottom.. At least an inch and a half. It should be enough to hold some stems of bamboo in place. Otherwise, you might need to pile some river rocks around the bamboo to hold it in place.
If you can't take care of them, I wouldn't do it.. but good light, cycled gravel and a live plant, I don't see any problems. (I've mailed fish, and they're exposed to worse enviroments.)
If there's not enough light, then instead of gravel, use a mixture of cycled gravel and ammo-carb. They should absorb toxins for the few hours you need.
Nothing will be more depressing than a bowl full of dead fish at every table... Consider the worst case scenario before you decide. If a fish dies, then the rest could easily die within an hour or two from the decay.