...and I found it. It's Windows-only sadly but it works fine in my XP Boot Camp VM. It's called "HDTVtoMPEG2" and can be found here: http://www.midwinter.com/~bcooley/
It's a tiny little app that you just unzip and drop anywhere. The version I got was 1.07 and it was simplicity itself to use - click 'Add', pick file you want, click 'Convert' - done.
To test it, I converted a 10 minute snippet of Endeavour's take-off and subsequent orbit of the earth. The file was 1GB and HDTVtoMPEG2 processed it in less than 5 minutes. I didn't hold much hope - especially when the resulting MPEG wasn't playable in either QuickTime or the Old Faithful VLC - but I tried streaming it to the PS3 and it worked perfectly!
All in all and handy little app for yet another quirk surrounding video streaming...
