So you're moving the start position of the video so that you're effectively adding black video for a bit till you want the video to actually kick in? Wouldn't you typically edit the video to be the correct length first, or don't you know what the correct length will be as you haven't actually scored the music yet?
I've had a look and it doesn't look like there is any way to do what you want. But syncing up shouldn't be an issue. You use the start time feature here (currently starting at 1:01:000):
Then you know exactly when it starts and you can use snap settings to line your audio up to that perfectly to that marker (or wherever you move it). Now in the case of you cropping the start shorter, you'll just have to do some maths and slip editing to get the line up perfectly. I'd suggest line it up first, then slip edit the audio (unless you are cropping the video in which case I'd suggest cropping the video, but also moving the start time forward equal amounts to the crop, so its relative position doesn't change and then syncing the audio, slip editing it, and now you can shift the video start time and move the audio accordingly without screwing up lining) Unfortunately, it looks like it can only be set to display in measures so you're best to only do your work in measures, or at least for lining things ups.
Thinking about it more now, I fully understand your issue I think and how it could get very frustrating. I'd probably suggest trying to get the video length perfect in a dedicated video editor FIRST, and then move it into SONAR as is so you don't ever need to move stuff around, if that's at all possible.
As for feature request, seems worthwhile, but I don't like your chances. I think it'd take a bit of programming to get something like that in with the current design of SONAR and video is NOT high on the list of priorities...