I don't think the ZF box has a dipstick. At least not on the Land Rover specced boxes. INEOS may have insisted on that in their production version though, if they have then I'd be very happy...
Both Mercedes and BMW are now starting to design cars so they can extract oil from the top by way of vacuum pump and not using sump plug. I currently own a Mercedes and that is how you are supposed to drain your oil, cleaner and more efficient.
see mercedes note:
SUBJECT:
ALL MODELS
OIL EVACUATION EQUIPMENT
Mercedes-Benz engines are designed to allow engine oil extraction from under the hood via the dipstick tube. The dipstick tube has an enlarged cross section and a formed extension at the top end. On the other side it ends a short distance from the oil pan bottom. Engine oil can be extracted using the dipstick tube.
Engine oil drain plugs will continue for the time being, although access to them may only be possible with the removal of engine compartment trim or noise encapsulation panels.
*********************************************************************************
Date: March 2001
Order No.: S-SI-58.00/118
Supersedes:
Group 58
SUBJECT:
ALL ENGINES
APPROVED OIL EVACUATION EQUIPMENT
The purpose of this Service Information is to inform dealers that Approved Oil Evacuation Equipment, which is used to extract used engine oil from the engine via the oil dipstick tube, is now available for order. Mercedes-Benz engines are designed to allow the extraction of engine oil from under the hood via the dipstick tube. The engine dipstick tube has an enlarged cross section, plus a formed extension at the top end where the dipstick is inserted and the approved engine oil evacuation equipment interfaces. The opposite end of the dipstick ends just short of the oil pan bottom, thus engine oil can be extracted via the approved oil evacuation equipment by using the dipstick tube. Additionally, the engine dipstick tube itself is the conduit through which spent engine oil is moved to the oil evacuation equipment.
In addition, service and repair components for existing approved oil evacuation equipment is also available.
Note: Because the engine oil dipstick tube is the conduit through which spent engine oil is extracted, inserting tubular probes through the dipstick tube is NOT recommended.