How to use querystring parameters in the databind section of ASP textbox in SPD 2007?

Question

I'm currently trying to find a way to dynamically set a field in a "New Item" DataFormWebPart SPD 2007 ASPX page which writes to an XML file, via a querystring passed to the page.

Ideally, it would work something like this:

Page would be opened like this:

../newitem.aspx?SerialNo=1234

Then within the page, the textboxes which write to a child node of SerialNo would create a new item under SerialNo "1234"

The XML schema looks something like this:

Serials

------Serial

         <SerialNo>1234</SerialNo>

         ------Items

               -----Item

                   <ItemName></ItemName>

                   <ItemAmount></ItemAmount>

Basically I'm trying to create item items via TextBoxes for itemName and itemAmount, under the appropriate SerialNo.

Is it possible to do this by using a variable in the databind section of the textbox?

i.e.

asp:TextBox runat="server" id="ff1{$Pos}" __designer:bind="{ddwrt:DataBind('i',concat('ff1',$Pos),'Text','TextChanged','','/Serials/Serial[{$SerialNo}]/Items/Item[new]','ItemName')}" />