1. Use adhesive, somthing good not something that gets hard. Dow Corning 732 is the best silicone I have ever used its awesome.
2. Screw through the face. At one time I was all about blind mounting stuff, then I had a couple jobs where I realized that there is no substitute for hardware. You only have 3/4" to get a screw in to hold that sign on, thats not very much in a windstorm. I personally really like the plastic caps things you were talking about. I'd use adhesive and put screws into the posts with the plastic caps, and Id never worry about the sign going anywhere.
Kevin