Yes.
To answer your original question. It does seem to work however you're most likely going to have an issue in NTSC land where the TVs just might not playout the menu if it was PAL.
Conversely the el cheapo but very good DVD player (XMS) that I have does it's internal menus and splash screen in NTSC.
So if you can make the menus NTSC, and just have a menu item for PAL.
Having said all that there's not much gear left in PAL land that will not play NTSC from my experience. The res drop is rather noticeable though and much of my DVDs for corporates gets played out on PCs anyway where it doesn't matter at all.
Try to find out what the user really wants. Most likely he wants video that plays in both NTSC and PAL land, and that is easy to make. Just make an NTSC DVD and you are good to go. I will go as far as to say that all players in PAL land these days play NTSC.
If he really wants a PAL DVD your best bet is to go two DVDs.
I will go as far as to say that all players in PAL land these days play NTSC
I think you are right - we have a lot of commercial DVD's in Denmark which are NTSC to buy, and I have not seen a newer player in Denmark that would not play back NTSC.