The best method I know of would be to open the HD video through AviSynth using DGAVCIndex and create a simple script to resize to 720 x 480 (using Spline36 resize or one of those) while encoding as a 16:9 m2v with HCEnc, or another encoder of equivalent or better quality that accepts .avs scripts.

If using AviSynth isn't something you want to get involved in, I might actually be able to do this for you; the only difficulty I foresee is that DGAVCIndex requires the raw h264 stream demuxed from everything else, so I'd first have to find something to do the demuxing of AVCHD on a Mac. There's also FFmpegSource, which is supposed to be able to able to open h264 in container formats, but that hasn't worked for me yet.