Forget my last comment, it was quite innocent. I revisited the .vh.sf documentation. Was thinking more on some eventual variable to be set in the .vmx file that would prevent the -flat.vmdk files from being locked, as the observed behaviour from part of my client would leave a door open to that.
I spent some hours playing around with the .vmx files provided by my client, no joy so far. Maybe the VMs that allow access to these files are just behaving in a buggy way themselves. In any case, being this lock something done on purpose, any still open gap would soon be closed as well.