Comment by Sanzig

IIRC, Dyson proposed using a thin layer of oil on the surface of the pusher plate that would get vaporized with each shot, but would prevent the plate from ablating away. This effect was discovered by accident during nuclear testing when oil contamination on metal surfaces in close proximity to the explosion would protect them.

Of course, depending on how much oil you consume for each shot, you will degrade your effective specific impulse - I'm not sure by how much though.

The other issue which you can't really get around is thermal, that plate is going to get hot so you'll have to give it time to radiate heat away between shots. This may be less of a concern for an interstellar Orion since the travel times are so long anyway, low average thrust may not matter too much.