Juicebox uses one set of configuration options for both normal and fullscreen modes and, unfortunately, it is not possible to change configuration option values once a gallery has been loaded.
If you want to change a configuration option value, you would need to reload the gallery afterwards and a gallery cannot be loaded in fullscreen mode (at least not easily and without problems).
The best I can suggest is that you post your suggestion in the Feature Requests forum thread.
This keeps all the ideas together and ensures that they are not overloooked.
Incidentally, the correct syntax for running custom JavaScript when a gallery has been expanded (or closed) is as follows:
jb.onExpand = function(expanded) {
if (expanded) {
// Gallery has been expanded
// Custom JavaScript goes here
}
if (!expanded) {
// Gallery has been closed
// Custom JavaScript goes here
};
}