The DVD spec allows subtitles to be on or off. Even if you set them up as turned on, and provide no option for a menu to turn them off, all a user has to do is to use their remote to turn them off. I see no option in Encore for remote subtitle action to be restricted.
You need to take the project back to Premiere, add the subtitles there, and make them part of the video. What your client wants are not really "subtitles" as provided as an option on DVDs. Well, they are subtitles, but the restriction you want is no possible; at least not in Encore.
I don't know if scripting allows this, but your solution is straight forward.
The only problem: I don't think Premiere provides an option for importing a subtitle track (you can export from Encore if you added the subtitles there).
