Just tell the officer you are here for flag-polling to get an open work permit on the basis of a spouse's work permit.I agree with you and wouldnt hesitate in trying it.
For flag-polling, should we apply first and go there or we can also apply on-site after getting consultation with visa officer to confirm the eligibility?
Then may check your spouse's open work permit and let you know whether you are eligible or not.
Don't forget to carry documents like a pay stub, job offer letter and description.