Thanks Kevin. I've read this a thousand times on the forum, but it didn't register that it was also true of file opening.
So I realized that the network drive I was trying to read from was mounted on my PC where I am using designer, but not on the server that hosts Ignition. So with renewed hope, I mounted this share on the hosting server and still nothing. Can't read it, and it doesn't exist according to system.file.fileExists().
I can ping the share from the server, and I can read the file from a command window, but from Ignition I can't. Ignition can see a file that I store locally on the server, but I can't see it on the network mounted drive.
This is a Windows server, and I am logged in with the same credentials that Ignition is using.