I just got back from the dealer (mercedes-Benz Cary, NC) and I had a chance to speak to the technician who replaced the desiccant in my car. He said it was a nasty job. You do indeed have to drop the plastic underbody tray, then reach around some frame members to access the desiccant access hole (T90 bit required). He also happened to have a battery sitting in the shop to show me. (It seems he just replaced the battery in another B-Class!)
Cost of the desiccant cartridge from M-B is $220 ("Air drier" Part No. 242-340-08-01), labor $100.
In the photo below the front of the car would be to the left. The T90 for the desiccant access is on the left.
The battery and a motor he had already shipped off were replaced under warranty as it was within the 8 year/100,000 mile warranty. The crate had Tesla logos on it.