It depends on whether or not you want to be use third party tools. Our company has a free product called Smart List Lite, which can give you column permissions capability, which is what you need.
If you cannot use third party add-ons, I would do the following:
1. Make the field read-only in the content type of the item
2. Create the same fields in the content type of the task spawned by the WF
3. Create a second WF that runs when the task is updated and copies the field values from the task into the original item.
I believe all of this can be accomplished using SPD alone.
Vladi