If you make subtitles in FCP they will be permanent.
Use a San-Serif font such as Helvetica or Arial with a thin, dark border to keep them readable. In FCP the Text > Lower Third generator is the best option as these are automatically centered.
Also consider shortening your text without losing the context. No need to have the subtitles reflect the script word for word. The viewer should not have to spend all their time reading.
Subtitles in DVD Studio Pro use a text document as their basis, in which you specify the font, size, color and timecode based in and out points. You can provide a button to switch the diplay on and off, or have them permanently on (Force Display).
Here is an example of a subtitle file used in DVDSP:
$FontName = Arial
$FontSize = 65
//The following subtitles are for scene one.
00:00:12:04 , 00:00:14:12 , Lemurs are the bullies of the wild.
00:00:16:14 , 00:00:19:08 , Here we see them bring down a moose.
Lots more information in the DVD SP User Manual -just do a search for "Subtitles".