It's so nice to know we're not alone with GFCI problems!! While we don't have a ton of customers calling with GFCI problems -- we seem to have 1 or 2 problem children every year. And we also keep cords off the ground, use silicone, etc. My husband went to one of the problem houses last night and sat in front of the house. It was a rainy evening and he had a hunch the pesky GFCI would pop again. Sure enough it did. He was able to track it down to the mini-lights on their small trees -- incandescents. We unplugged those and will be switching them to LED. That particular customer also has blue LED minis on different trees -- no problem. My suggestion -- try to be at one of these problem houses when the GFCI trips...then immediately start tracking it down. You should hopefully be able to narrow it down to one particular product, one tree, etc. Try to keep the house lights on a completely different circuit so at least if the ground stuff trips, the customer can still have the house lights on. That makes 'em feel a little better. We only have this problem at houses where there's stuff on the ground...the houses with just lights on the house and/or starbursts in trees...no GFCI problems.
Like someone else suggested...I'm dying to switch out GFCI outlets to non-GFCI outlets at problem houses. Is that legal? Do you open yourself to any liability? I would think you would want to get the customer's permission in writing.