I know that cell editing with datepicker is possible because of references here and here. However, when I click on the cell, no datepicker shows up. Below is the colModel en
I had the same problem with passing a function with PHP's json_encode, which is impossible but if you are using Zend Framework you could use Zend_Json_Expr('function ...') to use a function and then encode with Zend_Json::encode($var).
Still this won't solve the problem cause the event will not be fired if you insert it later thru AJAX.
You could have a look to Pike_Grid to see how it's done there.