you might be able to do this by creating your DVD in Encore (or any other DVD authoring app'), with the NTSC/Pal menu in place, but substitute the NTSC content with a PAL clip (or vice-versa), so the menus all work fine,
then build a DVD folder of your project,
then use something like PGCedit, (free from: http://www.videohelp.com/~r0lZ/pgcedit/index.html) to swap the Pal content for the NTSC content that you want to include,
I haven't tried this, and I could well be overlooking something very obvious, Maybe DVD's have to be one or the other, I don't know.
More research is required I think, PGCedit might be a good place to start though.
Panasonic SD9, Panasonic NVGS-75, Canon MV600i