And exactly how will it be played? That's all that matters. If it's bound to a specific playback device or format, then it will probably require moving with in the HD specs. If it only requires desktop playback from a PC in fullscreen mode via a standard media player software, then an arbitrary resolution should work just the same. Still, even then it's probably better to work in a standard full HD comp and only scale the output on encoding. You know, your client could change his mind and want this on a full HD screen, after all and then you'd be in all kinds of trouble having to redo it...
The content will be played on on 32" ad player in the mp4 format with the 1366 res. So the best idea would be create at 1080, like you said I only want to do it once.