The multiple update server behavior will allow you to update more than a single record, it is based on a recordset that you have on the page. One way that I could think of to do this would be to use the DataAssist Search server behavior to limit the records to update. You could do this by having a page that has all of the records listed, then have a checkbox next to them like you mentioned. You would want to set the value of the checkbox to be the id of the record and make sure to add in brackets after the name of the checkbox.
This page would post to your multiple update page. You would apply the DataAssist Search server behavior to the multiple update page and specify the search criteria as a list type that looks at the id column. This way when you post the id's from the checkboxes to this page the recordset will only contain the records of the id's you passed.
You can then make the updates to these records and click on your update button to commit the changes. This is just one approach to this with DataAssist. One of the best things about using the WebAssist tools is that you can combine the server behaviors to create the experience that you would like. While this specific setup is not a single supported feature of DataAssist I can help you out if you have any trouble with the different parts of this. Please post back with any questions that you have about this.
