You could accomplish this by the push of a button, the Edit Layer B button, which selects Layer B. You have to press the Split button to go to from split to single mode anyway, just press the Edit Layer B button after that. Or, with a later OS version (can't recall which one fixes it), you could save the patch with the Edit Layer B button already selected, but I'm on 1.1.4.5, so I can't really verify that. But the feature has been added somewhere along the line (if I recall correctly), so in a future "Main" OS version it should be possible.
-Just re-read the thread and realized that's what it was about to begin... So OS 1.1.4.9 and up should have this function?