Sorry if this is a little late....
On the International version of the S3 (GT-I9300) running JellyBean 4.1.2, you can enable camera functionality during calls by editing the file:
/system/csc/feature.xml
Look for the line:
<CscFeature_Camera_EnableCameraDuringCall>false</CscFeature_Camera_EnableCameraDuringCall>
Change the "false" to "true".
Unfortunately, you will also need root access to remount /system as read-writable in order to edit the file.
Just tried this on my Sprint GS3 running 4.1.1, and unfortunately it still doesn't work, even after saving the change and rebooting.Oh well, it's not something I'd do often, but it's a nice to have. I'll probably be updating to 4.1.2 soon, so I'll try again then.