You could use InnerHTML. It works in IE, but not sure about Netscape.
Example:
function checkfollowup()
{
divNAME.innerHTML = '<input type="input" name="field1_in" size="12" value"na">');
}
Then call your function from your link, or use an onChange or onClick.
And have your div named to whatever "divNAME" is above.
<div id="divNAME"><input type="input" name="field1_in" size="12" value"someothervalues"></div>