In general, Scaling and Root Planing is done by quadrants or halves. You will be able to chew on one side, and they will (should) use topical anesthetic. Don't be afraid, though it is natural, but the dentist/hygienist will be as gentle as possible.
Ask the dentist and hygienist about the procedure, and mention to them about your dinner plans. Also, let them know about your anxiety about the procedure (which is minimally invasive, at best). The topical anesthetic is there to keep you comfortable, as the hand instruments can put a bit of pressure on sensitive (possibly inflamed and more sensitive than usual) tissue.
This is something that is routinely done, but it is odd that he didn't mention your gingival health before scheduling a scaling and root planing appt. Be sure to discuss the issue (if any) before the procedure. That way you are in the know and both of you are happy. I'm sure he/she's just trying to keep you healthy. It's rare (at least I think so) to come across a dentist who is so periodontally involved, and it's a good thing he is so perio happy.