Comment 2 for bug 1470278

If you think allowing scripts to register their own COs which would be exposed to the GUI is the best way to handle this, sure. If this system is flexible enough to allow mappings to specify their own GUI for changing these, this should be easy for script writers and done automatically for simple cases. For example, scripts should not need to specify their own GUI for boolean variables; checkboxes should be made by Mixxx automatically. Similarly, for numeric variables, a number input box with up/down arrows would be made automatically and scripts could specify a range of acceptable values.

Related to Bug 1166016, see the wiki page I started recently for brainstorming about a new mapping format:
http://mixxx.org/wiki/doku.php/new_control_mapping_format