The tilde is "~" and the grave is "`". I actually hotkeyed this by editing the configuration file. The game showed it as hotkeyed when I launched it afterwards but it doesn't register it at all as a button press. It's a bit odd!

One alternative is to set up all your hotkeys such that they're moved all one to the right. This may take some adjustment but some people prefer it. Think of WASD for example, where it could be ESDF instead in FPS games, giving access to an additional column of keys to the left. The same could be true in any game! I'd probably not like tab being so far away, though. My pinky isn't very good!