As long as the fork is only used in-house I'm the customer. I do have access to my own source code. GPL is satisfied. I might as well create a closed fork and hand over the code to the people that buy my product. I can't place any restrictions on what they do with the modified source, but as long as I give them access to the source as required by the GPL that's fine as well. If they choose not to redistribute - fine as well. I might even create a closed fork for tinkering and not do anything with it. It's not the act of creating a closed fork that violates the GPL. It's redistribution without source - and that's independent of wether it's a fork or the original version.