If the reserve really is to protect the battery, then I suggest sticking it under the bonnet, or perhaps in the boot (presumably where the battery is). Stick it under a cover with "FOR EMERGENCY USE ONLY" on it.
If abuse is a serious worry, stick a bright red counter next to it that decrements at every push. When the counter gets to zero, you need to take the car to the shop where they'll do some rediculously expensive "reserve capacity recertification". Until you do, no more reserve-tapping.
ISTR BMW did something like this with one of their models. They had a special "sport launch" mode you could activate while pressing the brake while in drive. Once you released the brake, the engine would rev to several thousand before the transmission would engage, hurtling you forward at levels of acceleration approaching acceptable. The mode could only be activated a fixed number of times (10?). I don't believe there was any indication of a counter, at some point it would simply become disabled. If it was possible to reset the counter, I imagine it was forbidden at a factory shop, and considered a warranty violation if performed elsewhere.