Here's the answer from Kent at PG Support.

Hi Frank,

I noticed this a while ago. Unfortunately, there isn't a way to render a one-bar intro at this time. We're aware of the issue, and it's just a limitation of the program for now.

If you render the two bar intro, you could open the WAV file in an audio editor (or even RealBand), and delete the first half of the intro.

I'm sorry that I don't have a better solution at this time.