On vacation, but thought I'd comment on this topic before getting on the Harley to go run some errands in the cold Pacific Northwest. As pointed out in someone else's recent blog post, MPEG-4 can leverage what is referred to as a Sample Aspect Ratio (SAR)...not to be confused with Storage Aspect Ratio (SAR) or Signal Aspect Ratio (SAR). It's important to note that in the case of MPEG-4, the Sample Aspect Ratio is the Pixel Aspect Ratio (PAR); they are one and the same.
It's also important to note, again, that regarless of any of these numbers, the shape of the samples from an analog source ARE NOT DEFINED BY THE NUMBER OF LINES.
Oh, one more thing...most multimedia NLE and encoding applications provide precise control of all of these settings. Just an FYI. All the best my friends.