This is a very simple fix which will probably be included by default in later versions of Sauerbraten. It simply adds a few lines to your keymap.cfg file, which lets you use more than three mouse buttons (if you have them).

How to use:
Where "SAUER" is the directory you have Sauerbraten installed:
1) Replace SAUER\data\keymap.cfg with this file.
2) If you want to be able to bind your extra mouse buttons through the in-game menus, use NINJ4's Pro Config, or add the following line to your SAUER\autoexec.cfg file:
keys = "MOUSE1 MOUSE2 MOUSE3 MOUSE4 MOUSE5 MOUSE6 MOUSE7 MOUSE8 MOUSE9 MOUSE10 MOUSE11 SPACE LCTRL RCTRL ALT TAB ESCAPE BACKQUOTE SHIFT BACKSPACE ENTER INSERT DELETE HOME END PAGEUP PAGEDOWN A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7 8 9 0 F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 F11 F12"