You know that there is only one security mod, so whatever you apply will be applicable to both models.
Could you add a summarise level / use an existing summarised level to only allow high level to users who need to see figures in your plan model, and down to zero level for workforce?
See recent discussion => Data Security
Robert already provided you detailed guidance which you would want to review to determine if it best solves the issue at your company.
You may also want to go ahead and open an Oracle SR and file an Enhancement Request for security by Plan Type.
Good luck and let us know what you finalize on.