Ok, got the automation snapshot working! I was mistaken when I said "new song". I was using a song template I had created for B4II. It must have contained some previous automation.
With a new song using the Normal template, I can create automation snapshots just fine. And they work perfectly with Rapture. When I select a patch in Rapture, then create the automation snapshot, all the parameter values in Rapture are recorded in the snapshot correctly.
That's not the case with B4II. After selecting a patch in B4II and creating a snapshot, the parameter values are recorded at the middle position. So if Upper Vibrato is set to OFF in a patch, it gets recorded by Sonar in the middle which causes it to turn ON. But if I adjust each parameter in the B4 II UI before I create the snapshot, the values get recorded properly. Looks like a quirk of B4 II, and since it's been discontinued I'll learn to work around it.
Thanks for everyone's help!