I've never heard of a properly attached sparrow strainer coming off.IMHO they are a kludge for the wrong elevator moment. Ok on a prototype, but for production, you should probably develop the right elevator. Bits that are fragile and fall off have no place on an aeroplane. Especially if they are likely to go through the prop... I believe the quickie was the last Rutan design to use them. Could Marc or our other canard experts comment on later Rutan elevator feel?
And they are not required. There are a number of Velocity's flying without them. They are there only to relieve the amount of force required to control the elevator IF the trim spring breaks. And even if the trim spring breaks and you have no sparrow strainer, the pitch can still be controlled. It just may require more force.