There are things like the “Open Digital Rights Language” being worked on, which may apply in this area:
Because access control in SOLID (and really at Linked Data and really web level) is at the granularity of files / documents (not fragments of files / documents), then it may or may not make sense for digital rights to be applied at same level.
So it could also appear to be possible / feasible to grant a person or app or etc access to a file or folder on your SOLID Pod, which would then contain embedded / inline or as a separate file containing descriptions of other files / folders specific digital rights.
And I agree, access control is not the same as digital rights, and GDPR type concerns regard digital rights, not mere access control.
Being granted actual access to digital resources (such as even a public file / folder within a SOLID Pod) does not imply digital rights of any kind … and similarly just denying access to any file or folder also does not imply any digital rights.
ACL being access control, not Digital Rights.
So similar to a Github repository, and files and folders within, there needs to be specific licensing information made available for the contents.
This can done however the owner wants, but would certainly be valuable to describe in RDF (using something like ODRL possibly).