A bit of an update...
I put a call in to the tech support people (well, Kevin) at Hellwig. I told him what I thought was going on and also that some other owners seemed to have solved the problem by torquing the u-strap bolts to 65 ft-lbs.
When I mentioned the 65 ft-lbs he immediately put me on hold and went off to talk with an engineer. When he came back on, he said that engineering had told him that 50 ft-lbs was ok, but that they recommended against higher torque numbers. Your mileage may vary.
I also told Kevin about an assembly compound used for some carbon to carbon interfaces on bicycles. This material is essentially a grease with a gritty material imbedded in it. The grit grips both sides of an interface (like a seat post in a seat tube) so that they can be held in place with lower torque values. Kevin seemed to think this was an ok thing to do - I've got some of the compound - here's a link to it:
But I'm also thinking of just mixing some fine sand into a grease and applying it lightly between the u-straps and the mounting area on the frame - I'm thinking sand would be more aggressive than what's used on bicycles. (The grease is just to hold the sand in place while the bolts are being torqued.) I'm hoping that one of the grit approaches will allow me to hold those straps in place while only torquing the bolts to 50 ft-lbs.
Anyway, I haven't done any of this yet, but will report back when I do.