The comment above was the breakthrough I needed to get moving on actually opening ClickOnce associated files. Of course I was trying to modify Program.Main() to accept 'string[] args', and the debugger was complaining:
"The current project settings specify that the project will be debugged with specific security permissions. In this mode, command line arguments will not be passed to the executable. Do you want to continue debugging anyway?"
As it will, if you have published your ClickOnce deployment. Hopefully some of this description will help others find the tip above more quickly than I did.
My added note is that ActivationData is a sting[], and hence tempting to assume it passes an array of file paths, but they are actually in URI format. You'll need something like the following:
string[] args = AppDomain.CurrentDomain.SetupInformation.ActivationArguments.ActivationData;
if (args != null && args.Length != 0)
{
try
{
Uri uri = new Uri(args[0]); // SDI only allows one file
if (!uri.IsFile)
throw new UriFormatException("The URI " + uri + " is not a file.");
newFile = !OpenFile(uri.AbsolutePath);
}
catch (UriFormatException)
{
MessageBox.Show("Invalid file specified.", Program.Name);
}
}