The way I understand Crop Mode is as follows:
In LiveView, you press the magnifying glass which will zoom in 5X (10X if you press it again).
Press the trash button to go back to ML, in the Video menu select your desired aspect ratio and resolution. You will find that while you are zoomed-in, you can now select much larger resolutions (but you will still be limited by how fast the data can be written to the card).
Exit ML and start recording. The new video will be recorded in those higher resolutions.
The main problem as I see it is that there is no way to accurately monitor what you are actually recording since there is a discrepancy between the zoom in LiveView (5X) and the data being recorded in Crop Mode at 3X. Is there a way to see what is being recorded in Liveview accurately?