I *think* you can just provide the PRJ file without a catalog. They won't be able to make any changes to the PRJ contents (since they don't have the catalog), which can be a problem - everyone has their own instruments, supports, and the like and they have to be added to the catalog.
Or, use the catalog inheritance system to create a catalog for that client and build the PRJ off that catalog.
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If you are going to fly by the seat of your pants, expect friction burns.