I don't think there is much you can do. Our local theater had the same problem with A Tuna Christmas. They were not able to get rights and they were several weeks into rehearsals.

That is one of the reasons that the royalty houses guard the copyright so vigorously. It really is a Catch 22 - if you try to get rights more than six months before the performance they give you a provisional approval and make you apply again within the six month window. Less than six months and your season is set and then you can't get rights. The same thing happened to me when I tried to get rights from Dramatists for The Vagina Monologues just a few weeks ago.

On the positive side, I know of a case where the theater was successful talking the royalty house into giving them permission. In this case the play was restricted because a major Seattle theater was doing a production of the same play around the same time. The local theater was able to convince them that they were far enough away that the restrictions should not apply. Since you are already in rehearsals it is worth a phone call to see if you can change their mind. (My gut feel is that you should not mention that you are already in rehearsals.)

We faced the same problem with a production of "Ladies of the Camellias." They granted the rights, we cast the show, and then our group received a letter saying rights had been withdrawn because a professional company had optioned it, and they had first dibs over us. Since we're within a 60- or 70-mile radius of Baltimore, Philadelphia, and D.C, there was no telling who had optioned the show, but whatever contract they'd negotiated had stated that no other productions were to be performed in their area until they'd done it. To replace "Ladies," we wrote an original readers' theater comedy piece that used not only the actors who had been cast in the original play, but the ones who had auditioned and hadn't gotten parts. It turned out to be a bigger hit than anyone expected! Oddly enough, I don't think anyone around here ever did "Ladies of the Camellias," so the publishing house definitely lost out on that one. Several years previously, we'd had the same thing happen, and went with a production of "Love Letters." If you're really short on time, readers' theater might be an option.