Would making the site visit mandatory be against any regulation and potentially cause protests which may be upheld? Would this be considered restricting competition?
Yes, requiring a site visit could create problems. The time to alert potential BPA holders of the site visit issue would have been when the Navy solicited and awarded the original BPA, but presumably this is not included in the BPA terms/conditions. One possible solution is to include a note in the RFQ that you will be available for any offerors who want to visit the site in case they would like to see for themselves the access, measurements, etc., without indicating that offerors who do this will have a competitive advantage. Another idea is to take the most relevant photos and measurements yourself and include them as attachments to the RFQ. In any case, you should run this by your activity's legal counsel since you are concerned about implications for potential protests.