Just to give some figures, a lethal blood alcohol concentration is 0.6 % (or less). I doubt that urine could become more concentrated than this, because it would take some time for the kidneys to process the blood, and you’ld be dead before that.
The minimum proof recommended to flambe food is 60, or 30 percent, which I assume is close to the minimum concentration of an alcohol-water mixture that would be flammable.
Therefore the minimum concentration of alcohol needed to produce flammable urine is about 50 times higher than one that would kill you.
Giving yourself a “urinary tract enema” with vodka would probably do the trick. I bet that’d hurt A LOT ! Probably make things shrivel up and drop off too. Guess it all depends on how badly you want flaming pee.
No, really, I have. I brought along a small can of Ronson lighter fluid on a camping trip, and then waited for the proper moment when everyone else was away from the fire but could still see me. I gave everyone a side profile, unzipped my pants (but did not let the sea otter out to play) and squirted most of the contents of the can onto the campfire. Big flames, big laughs.
But it won’t exactly get you into Oxford. I shouldn’t have to tell you not to try this–it’s alarmingly unwise, because the flame can creep up the stream, and you know where it’s being held.
This has to be a fairly old maneuver since I learned it from my father, who was a fantastic role model, let me tell you. My guess would be that some joker decided not to let anyone else in on the trick one night, and the story got around.
My uncle did something similar back in the 70’s. Only with charcoal lighter fluid. The fire shot into the can, exploded on him, and ruined his polyesther leisure suit. And 72% of his skin, which suffered 3rd degree burns.