Happy to share that this is now possible!

My Rinkel on desktop:

Go to any call, and tap on the 3 dots on the far right, in the column "Quick actions". Then click the Delete button. Or, when you've opened the Call information window, you can also use the 3 dots in the top-right corner to delete the log.

Rinkel app:

Included in version 3.10.0 of the Rinkel app, which is being released as we speak.

To delete a call log, go to the History, tap the call you'd like to delete, and then click on the 3 dots. Tap the delete option.