For a special education student (HFA), what are the rules regarding observation of potential proposed placements?

Yes a parent should be permitted to observe any proposed placement. These observations are not put in the IEP. N.J.A.C. 6A:14-4.1(k): provides: The district board of education shall provide the parent with the opportunity to observe the proposed educational placement, including the general education setting, special class programs and out-of-district placements in a program operated by another district board of education or a private school placement, prior to implementation of the IEP.

I agree with Karen. the District is required to provide you with access. As to the details (e.g., times, etc.), I would suggest that you try to compromise with them, working with the district staff in good faith so that you get sufficient access to make an intelligent decision about their proposed IEP. Engaging an attorney is not something you should have to do for every small disagreement. Try to work something out that you can live with. If you can't work it out, or if they're being unreasonable (e.g., entirely denying you access) then definitely at least consult with an education lawyer.