When I click on an inserted Impress video, the video toolbar pops up. This toolbar has controls which let you advance the video playing position. Is there a way to begin the video at this position while watching the slideshow?
I don’t know of any way to achieve this in Impress currently (LibreOffice 3.5.2).
You can use some video editing program to crop the file before inserting it in presentation. Open-source cross-platform applications include Avidemux, Blender, Jahshaka. There are number of others, for example VirtualDub for Windows, which should run on other platforms using Wine.
You might also consider filing a feature request in LibreOffice Bugzilla, although something connected with this question but more broader in scope was already proposed as one of Development Ideas.
I was afraid this might be the case. However, if this is true, what exactly is the actual purpose of allowing one to navigate to any point within a video while in creation mode? There is also a Loop feature. It seems to me that it was intended to be used, but somehow never got finished.
The purpose is in showing a specific frame on the slide as long as the video is not playing.