"Allow Unassigned" option for forestry towers for harvesters

At times it's helpful to have tree harvesters be able to free roam and cut mature trees where they find them, but as soon as you assign one harvester to a forestry tower all other harvesters are forbidden from working that area naturally - this is a mechanic that only seems to exist for the forestry towers with no real way to override it.
It'd be helpful if forestry towers had an "allow unassigned" option much like ore sorters where any unassigned tree harvesters would be allowed to harvest from that forestry tower. This would reduce the need to micromanage (while still preserving control since it'd be an option).