Thanks for the response borate, but... no joy.
I've been messing with this all afternoon, both with VideoPad and Express Burn. Apparently what I would like to be able to do is not possible.
If Subtitles are set to Show, then they are integrated into the exported video and display on playback, but you can't turn them off. The reverse is true. When not set to Show, they don't appear and can't be turned on. This is true not only with a DVD export but also with a video file using Windows Media Player for playback. Both VideoPad and Express Burn don't export that functionality.
Now the interesting thing. Save the subtitle text file as .srt (not .ssa), and name the file exactly the same as your video. Include it in the same folder as an exported copy of the video where subtitles were set to not Show on export. Open the video with Microsoft Media Player and... you can turn the subtitles on and off.
Doesn't help me given I want to produce a professional style DVD.
HUGE disappointment. Why have the tools to create a professional style video and then not be able to package it?