I'm doing a series of exports of long sequences from FCP which I want to have a widescreen matte and text burn-in. Is there a way to add both of these things using the DVDSP subtitle function, in order to avoid long export times in FCP?

You want a Watermark over the entire video on your DVD? What is a widescreen matte?
You can choose a 16:9 frame size if the content is 16:9 material. A DVD subtitle is not a way to Watermark your product; best to apply a track above the video in FCP with the watermark before export.