Welcome to the RS forum, Jim!
I don't have any personal experience nor do I know anyone who hasn't reserved through the Vatican Excavation Office but ran across this:
https://www.viator.com/tours/Rome/Scavi-Tour-to-Tomb-of-St-Peters-in-Vatican/d511-115257P8
Now, I'm also not a fan of booking through Viator but the actual ticket vendor (See under "Additional Information") is listed as "Vatican City Tours and Tickets" and their supposed website came up as "This connection is not private" and possibly fake. I was unable to find anything else on them.
That said, there looks to be actual reviews from customers whom have purchased these tickets through Viator, and I suggest you spend some time browsing some pages of those. Do pay attention to the "admissions and booking fee Euro 25/person paid on site" that is in addition to the $120.00 pp ticket, and that it is expected to be paid in cash. That came as a surprise to some of the customers. Also stated is that the order of tour may be changed. According to the Vatican Excavations Office site, no one under the age of 15 is admitted to the scavi, and you are not allowed to take photos or video.
Again, I will not vouch for reliability/quality of the seller but, well, it's a possibility.