If in "Windowed mode" of "Key definitions and WinLIRC"-page you assign a key or mouse button and also select the box "Global key", the key you defined for windowed mode will also work in full screen mode (even if it does not show that key under "Full screen mode").
In other words a keydefinition under "Windowed mode" for which the box "Global key" is selected, also works in full screen mode.
A "Global key" being applicable is also indicated under "Full screen mode" by the box "Global key" also shown there as selected (but "greyed out" as "Global key" can only be selected under "Windowed mode").