Using Final Cut Studio, I've been messing around with some different ways to convert a commercial shot in 720p to the highest resolution SD 4:3 version possible.
Let me know if there is any quick, sure-fire way familiar to you that you'd be willing to share!
The best "sure-fire" way to do this and get perfect results is to down convert through a card like the Kona 3. The card does the down conversion in real time and is pristine. This is the process I use and am always satisfied. It's far superior to doing a software down conversion through Final Cut.
The real sticking point here is that you are pursuing a down conversion for broadcast and for broadcast it has to be perfect. In my opinion Final Cut just doesn't do a good enough job down converting for this purpose. If the down conversion was strictly for viewing on your computer Final Cut is fine.
But, that being said, the easiest/quickest way for you to accomplish a down conversion with in Final Cut is to copy and paste your 720 timeline into a Standard Definition timeline and allow Final Cut to render. I think you won't be pleased with the aliasing and artifacts you will likely see.
my last gig which was shot using hvx200 at 720p 24pn was converted to SD footage at 4:3. I opened a time line Seq to match the HD 720p footage unedited. dropped my footage, rendered out as QT self contained and converted to SD using compressor. Drop my converted footage into SD time line, edited, rendered, and put on DVD for delivery. results were good.