We have fairly good success treating dry wells with Septic-Scrub(TM). Generally the recommended method is to pump the dry well, agitate the bottom to break up the anaerobic crust, and add two bottles of Septic-Scrub the first day and two the second. I then recommend monitoring the dry well and when the water level rises to about 30-40% of the top, add another two bottles of Septic-Scrub. Our experience is that when dry wells are treated, then we are probably going to that to treat them in the future again. This is because the drainage area in your dry well is only about 100 square feet (about 75 square feet in the bottom and some small contribution from the sides). As you can see this is a "small" drainfield. This is one of the reasons that dry well are not generally an approved system today. 

In your case the disposal of some 5000 gallons of water can be dificult and expensive. I would still recommend that you remove as much water as possible before treatment. If this is not possible, a higher risk treatment would be to add four bottles of Septic-Scrub directly to the full dry well. If we get somewhat lucky, there will be enough reaction with the bottom to allow drainage. I have had many customers report that their dry well emptied over a few days. If this works, I would then recommend agitating the bottom and adding another four bottles to the system while it is still essentially empty. 

If you have any questions, please do not hesitate to call.