The rear wheel must be removed to access the rear brake pads. But there is a catch. In order to remove the rear wheel, the rear caliper must be removed. Removing the rear caliper is accomplished by removing the rear master cylinder, hose, and caliper as an assembly (while removing the rear wheel). Of course, to get the rear master cylinder off, the rear fender must come off. Nothing is easy.
The single most important spare part to have on hand is a pair of rear brake pads. You will probably go through rear pads twice as fast as front pads. It is also not easy to see how much friction material is remaining with the rear wheel installed. I love EBC pads and use them for all motorsports. According to my EBC contact, the X-suffix pads are supposedly gripper but I have not really noticed a difference. Below are some of the equivalents I have identified:
Rear Pads: Galfer FD224 EBC FA351, SBS 803, Ferodo FDB2127, Vesrah VD9003 (I do not recommend using the Jitsie / GoldFren 173. They are very hard and useless in my opinion.)
Front pads: Galfer FD223, EBC FA303, SBS 802, Ferodo FDB2109, Vesrah VD9001
Rear master cylinder rebuild kit: Braktec 853028MO0
Front master cylinder rebuild kit: Braktec M853001MO0 (newer style)
Front master cylinder rebuild kit: AJP M CYL RP KT 1007 (older style)
Although it is physically larger, I actually prefer the old AJP front master cylinder over the Braktec used on later bikes. To me, the brakes have a better feel with the AJP. It is possible the lever ratio is different as they both use 9.5mm pistons. If you are looking to upgrade the front brake, Braktec's Monoblock is a very nice caliper. I have only ridden the cast model, but imagine the much more expensive CNC machined-from-billet version is ever stiffer.
All brake pads wear tapered. If I am going to reinstall the same pads (after cleaning, for example) I mark the pads as inboard and outboard with a Sharpie pen to make make sure they go back the same way they came out.