The last documentary I worked on I simply used Boris Graffiti and created a sharp looking text, w/out a background to it. I've not used Boris FX, but I know in Grafiti you can utilize image layers; you might be able to create an image gradient and import it in as a layer in FX and scale it to fit behind the text you're placing on screen. The other option is to create an image with the text already on it that you import in as a layer on your project timeline. The problem with this can be dealing with the alpha channel as well as getting the alignment of your titles the precise way you want them. Its been a long time since I worked with Final Cut, I edit on Avid systems generally, so I don't remember how hard it'd be to deal with that image as a second layer and whether it'd be worth it compared to placing a text plugin and arranging things that way.
Hope that helps,
Christopher McNeely