Because that he remembered not to shew mercy, but persecuted the poor and needy man, that he might even slay the broken in heart. As he loved cursing, so let it come unto him: as he delighted not in blessing, so let it be far from him. As he clothed himself with cursing like as with his garment, so let it come into his bowels like water, and like oil into his bones. Psalm 109:16-18
But the Bible-god not only curses bowels, he also, on occasion, messes with them so much that they completely fall out.
Thou shalt have great sickness by disease of thy bowels, until thy bowels fall out. 2 Chronicles 21:15
So praying for diarrhea has a solid Biblical basis. And though the following video is just making fun of Focus on the Family's (failed) prayer for rain on Obama's acceptance speech, it might be worth a try.