I just saw that when I upload my binary to the board using J-Link I get the following message:
CSR access via abs. commands: No
This points to me this thread: Can't access CSRs on HiFive1 rev B from Freedom Studio (via J-Link OB)
If I interpret this thread correctly, that could mean that accessing the CSRs through J-Link is currently not possible because their firmware doesn’t support it. That would be a real PITA.