Cyclocross Rim Wearing with Rim Brakes and appropriate Solutions
Option 1: Mid-race bike change

Many cyclocross riders switch bikes mid-race due to build-up of mud and debris and the dirt emulsion which accumulates on braking surfaces. Ideally you would have an assistant to supervise your belongings in the pit/service area and rinse the bike you've returned to prepare it for another bike change later in the race.

Option 2: Thicker rim-walls

Some rims claim to have thicker braking surfaces. You may identify these based on the difference between the external rim width and the ETRTO bead width (e.g 622-17 indicates a 17mm bead width)

Option 3: Brake less

Improving your cornering at speed so you don't need to brake so much.