I'm using a 9700 on OS5, and the stock Precision Zen theme, that shows calendar appointments on the home screen.

I use Google Calendar integration so my calendar gets updated over the air, but the home screen doesn't update for a while (even though the calendar itself has). Is there a way to force a refresh of the Home screen?

There is no refresh for the home screen. You have to rely on the refresh settings for your calendar and wait for the theme to pick up any changes.