Knockout selectlist binding
From a once over:


Your indentation in ko.extenders.selectList = function (target, params) { should be 4 spaces to make it far more readable
write could use a ternary approach:

write: function (newValue) {
  target( newValue || undefined );
}

This code could use some more/better comment
JSHint could find nothing wrong with it
I am not too excited by seeing all those options as part of the HTML, it just feels wrong. None of the tools work for you in this case ( linting, beautifying, even colored syntax ). I guess that's more a reflection on knockout than on your code.