Elusien, thx for your response. Your method does remove the leading null video - I got the nerve to try it and was able to back out of it. However, the ripple left of the video track violates my stated constraint - that the relative position of video and audio clips must not change because the video is placed against features in the audio track. What is needed is to, in effect, perform a cut through both the video and audio tracks, both of which have null content at the cut point, then ripple delete the left-hand pieces, thus rippling BOTH tracks the same duration and maintaining relative position.
Ultimately I used my proposed workaround of generating a dummy video clip using Open Other->Color Bars then placed that clip (in OVERWRITE mode) at 00:00:00 to span the leading null video duration past where the audio track starts. Then the SPLIT tool will work as expected and do a multi-track cut. Then the left-hand pieced can be dropped with ripple of both tracks in sync left to 00:00:00, thus maintaining relative position.