Hello Martin, I think your logic is exact. The only problem that i faces trying to work with worksets through Dynamo is that Dynamo only retrieves some sort of numerical Id of the worksets. My way of working around this is:
-selecting a model element that is in the workset i’m interested in to retrieve the Id of this workset (circles in yellow in the image)
-I select and filter my elemets by parameter value (exactly as you said)
-I use the Id of the workset to set the parameter value of all the elements
there are probably more advances ways to do so but this is pretty simple and does the job. Hope it helps!
