Hmm, I didn't think they took reservations. You may want to not only check the calendar, but talk to someone at the restaurant. You may be surprised what acts bring in the crowds. An obscure rock band might have a significant local following. HOB does something called front of the line: Dine in the restaurant and you get to enter the concert venue bypassing the line outside. In our city the doors often open at 7pm so your 6:30pm could turn out to be a busy time at the restaurant.
The food and apmosphere are good so I'd go regardless, but just be prepared.