A new syntax has been added to '-mapOptionKey': #key code:character. For some characters, the client needs both the key scan code and the actual character. If you are having trouble mapping the desired key to the option key, you may have to use the new syntax.
For example, to map the § key, you would use:
-mapOptionKey=#167:§
This will consistently map the option key to the § keyboard character.
To determine the correct key code to use, in the 'debug' section of the client preferences is an option to turn on key codes: -showKeyCodes=true. This will display the key code when you press the key. This can be enabled/disabled after logging in, if desired.
The old syntax of java key name is still supported.