I'm putting out a SD DVD w/ HD digital copy...

What output codec and wrapper for the HD digicopy is most widely compatible or closest to being a standard... something they can toss on a thumb drive and play on their HDTV? I generally use h.264 .mp4, but I know some older windows machines choke on these and neither of my TVs will play them.

mp4 is correct. The choking is usually due to processor power than codec wrapper (although that is important). Experiment with reduced bandwidth and see if you can get an acceptable image with more universal playability.

I think a key element also is the bit rate of the file. For instance my Roku HD player will only play back a file with a maximum peak bit rate of around 12Mbs. An 1080p file exported at high quality settings for H.264 could have a bitrate up to 20Mbs.