You can probably use the Notes section from Workspace, but what about if you add your message to the task instruction. The instruction is shown on the card or the grid view in Workspace and is a nice place to put information for the next user.
On the route, that goes back to the originator, you can add a SetValue that populates a string variable that contains the instruction message and use that variable for the instruction property on the User step.
Let me know what you think.
Jasmin