A priest who is in good standing, and who can present a letter from his bishop (dated no more than a year ago) can’t be refused to concelebrate, unless the host priest has some reasonable doubt as to the priest’s legitimacy.
Can. 903 A priest is to be permitted to celebrate even if the rector of the church does not know him, provided that either he presents a letter of introduction from his ordinary or superior, issued at least within the year, or it can be judged prudently that he is not impeded from celebrating.
You make a good point that the “ts” must be crossed; That includes canon law above all else (except Divine Law), and that trumps any local policy (which must conform to canon law) and anyone’s personal opinion. Yes, dioceses do have policies on visiting priests, but those policies always conform to canon law, including canon 903, of which every bishop is certainly aware.
A visiting priest, who can show that he is indeed a priest in good standing, has a right to exercise his priestly ministry and to concelebrate. Common courtesy, of course, also plays a role here. For example I can’t simply walk into a church 3 minutes before Sunday Mass starts, present a celebet and demand to concelebrate–even though that would be within canon law, it would be rude, and so it’s not done that way. A courtesy phone call ahead of time, or some other means of introduction is always in order. Likewise, there are always special circumstances (like a papal liturgy, to give an extreme) when other procedures have to be followed, but those are unusual and not really what we’re talking about here.
Please understand that when you begin by saying “I would imagine that…” and then go on to present what you think should be the policy for priests joining in the sanctuary, that can cause a lot of confusion.