What you can do is play them in media player classic or VLC and choose to crop 4:3 in there on playback. Thats how I watch on my 4:3 PC monitor
Here's my setup
29" 4:3 TV
HVR-2200 Tuner Card
ATI HD4850 graphics Card
Output to tv: Component or composite. Right now i'm using Component. Get same results through composite.
Player: PowerDVD 8, Media Player Classic Home Cinema, DVBViewer, VLC media player, any Directshow player.
Video renderer: Overlay
Codec: Poweredvd8 or ArcSoft TotalMedia Theatre
Everything i was trying was based on overlay being displayed full screen on tv(secondary display) while pc desktop was free to use. What i've now realised is that pan-scan doesnt work in this situation. So now when playing these "Pillar Box" broadcast's i switch to "standard dual mode". That means in order to have video displayed full screen on tv it has be be full screen on pc monitor.
Now to do test's under this setup.