[dylan604]

Why do you say you can't do expressions like that on the command line? I do it all of the time. You have to escape the parentheses, but other than that it is totally doable.

[demomode]

You can't use expressions like that (from ffmpeg cmld) in gst-launch.

As far as I know in gst-launch you can't access properties from one element (video stream width) in other elements.