I decided that I would like to have some idea on whether we should park the GL450, or just ignore the pending recall. If you are interested in doing the same thing, here is what I did...
This is NOT a suggestion that you ignore the recall. Your car, your risk, your decision.
Get a good flashlight (torch), raise the hood (bonnet), and look in the extreme left rear of the engine compartment (LHD vehicles). There, you will see the brake fluid reservoir sitting on top of the master cylinder. The master cylinder is bolted to a large disk-shaped tin can, which is the vacuum boost reservoir/cylinder. The reservoir is made from two metal stampings, which are crimped together with the fabric and rubber vacuum diaphragm trapped between.
Note that there is a large molded shield under the wiper arms at the base of the windshield (windscreen). This shield is designed to divert the rainwater and washwater coming off the windshield, and keep it from damaging wiper system and the brake system.
Apparently, the hole where the left wiper shaft comes through the shield has a soft grommet around the shaft, and this grommet can leak. If so, the leak is directly over the seam in the boost reservoir. Just in case, the Benz engineers added a molded EDPM (rubber/plastic) ring around the seam to keep the water out of the seam. And, apparently, somewhere, somebody's GL or ML still got water into the seam, and the reservoir failed, resulting in a US recall. I have no idea whether there is, or will be, any need for a recall of RHD vehicles.
As I understand it, when the Mercedes dealer gets the vehicles in, the techs will peel forward the rubber ring, and see if there is any corrosion in the seam. If not, they will just return the vehicle and mark it off of their recall list.
I peeled the rubber ring off the top of the seam (there was no sealer of any kind), and found a pristine joint, with absolutely no evidence of corrosion. We plan to drive the vehicle until Mercedes calls us, and not worry about it. Rob, a good friend who also has a GL450, did the same thing, and also plans to drive the vehicle.
If you decide to make this check, look carefully at the seal ring before you touch it, and make ABSOLUTELY CERTAIN that you get it back exactly as it was before you peeled it back.