On topic first (don't worry I'll soon digress) would it need two elastic loops to keep it upright? My mudguard is held on with two large O rings so it stays aligned with the downtube. It stills get rotated around the tube sometimes but it stays in line with it.
Good concept though, would work well in the summer here and I guess all year in Wales. Here in the winter it would have so much mud on it that by the time you'd got it off the downtube you would be cleaner if you'd just dug the hole with your hands
As for Ralph, I wasn't aware he when to the toilet, I assumed he didn't - like the Queen - he also never carries money.
As for monkeys throwing their poo.... about 40 years ago the local adventure park (Robin Hill) had a monkey in a cage just at the entrance. He discovered how to keep himself amused by.. well he.... umm.... anyway, when he had reached the 'climax' of his entertainment he used to throw the... umm.. the... you know... at the cars stopped at the booth where you pay
He was something of a legend amongst the school trips. They eventually had to move him, and I assume intervene. as his 'you know' was redder than the teachers faces who were asked what he was doing 
