I know of no requirement within FAR 43 Appendix E as you indicated, but if there is a leak, then there is an error in your airspeed. It may be an insignificant amount, or it could be several knots. But I'm sure the FAA could make a case that the aircraft did not meet airworthiness requirements if they chose to be nit picky about it.
Now, when it comes time for your annual inspection, FAR 43 Appendix D will apply. It essentially states that all systems must be in operating condition with no defects. A leak is definately a defect.
A quick way to check for a leak in the pitot, and to test the entire pitot system, is to get a piece of soft rubber tubing, tygon is best, that you can slide over the pitot tube, leaving at least 6 inches beyond the tip of the pitot tube. It should be snug and put a tie wrap on it to hold it tight. Put a couple of pieces of good duct tape or aluminum tape over the bleeder hole in the rear of the pitot mast. This should be accomplished in a hangar or under an awning so radiant heat from the sun cannot cause an air expansion error. While someone is in the cockpit looking at the airspeed, clamp the open end of the tubing and then slowly start rolling it up toward the mast. When the airspeed gets to 160kts, clamp it off and let it sit. If it loses more than a knot or two over a 15 minute period and over two or three testing cycles, then you definately have a leak. Just make sure when you losen the clamp that you slowly unroll the tube so the airspeed bleeds off slowly and doesn't damage the diaphram inside.