How do I make $.serialize() take into account those disabled :input elements?

Temporarily enable them.

var myform = $('#myform');

 // Find disabled inputs, and remove the "disabled" attribute
var disabled = myform.find(':input:disabled').removeAttr('disabled');

 // serialize the form
var serialized = myform.serialize();

 // re-disabled the set of inputs that you previously enabled
disabled.attr('disabled','disabled');