As it turns out, the addition/replacement of the modal method was not the solution. Here's what I found. I'm passing in the sequence context and verifying its validity with the "Not a number/path/refnum" utility. I had the modal code inside a boolean case statement that would do the NotifyStartOfModalDialogEx if "Not a number/path/refnum" was TRUE. Well, the sequence context was valid, so the true case (incorrectly coded) was never being called. This gave the appearance that the NotifyStartOfModalDialogEx did the trick. However, I had to use the sequence context for another purpose and dropped the code into that same boolean case, and the code never got called. That's when I discovered I had the True/False cases reversed. When I swapped them, the new code was called (along with the NotifyStartOfModalDialogEx) and my HMI locked up upon completion of the test.
I took all the modal code out of this VI, and things worked just fine. I do have the "Show VI Front Panel When Called" checked, which apparently is sufficient in this case to ensure this dialog is modal.
