If anyone comes to this query with the same question, here is the definitive answer!
Gnome sets itself up, including the keyboard shortcuts, before running the user's start-up scripts. In other words, it can't follow the instruction to use F20 to launch Firefox because it doesn't know that such a key exists until it's run the script.
The idea that the extra keys need naming (e.g. as F13 &c), which I got from a previous post, is wrong. Just pressing the key will insert its scan code into the shortcut: e.g. key 230 is automatically entered as 0xe6.
For using xbindkeys to launch a program from a keypress when the desktop doesn't include that option, the same key would be called c:230. e.g.