My approach to this would be to make an AJAX call to an action mapped behind a JSON namespace (e.g., myApp/json/getObject.action) passing the value of the select field (e.g., ?mySelect=1). Use the response to populate the multi-select and/or text box using JavaScript.
I've never used it, myself, but you might look into the dojo plug-in. Of course, you don't need to use dojo. If you're used to another js library, use it. I use prototype (js framework lib) and scriptaculous (effects engine lib). I would also use the json plug-in for your responses. It's very simple to set up and use. It basically just maps your action as json.
Hope that helps get you started.
Resources:
json exampledojo & ajax exampleprototype & ajaxscript.aculo.us