Yes. onSubmit you can call a function that checks the value of your text box. if the value is nothing, you can assign the value of something, then proceed in sending the form info. here's half pseudo, half real code (in other words i've been working in other languages recently and not JS, check syntax).
Function checkTextValue(TextName){
if document.forms[Form1].elements[TextName].value == "" {document.forms[Form1].elements[TextName].value == "Not Specified";
}
}
<form name="Form1" method="post" action="scripts.asp" onSubmit="checkTextValue(NameOfTextBox)">
<input type="text" name="NameOfTextBox" value="">
<input type="submit" name="submit" value="Submit">
</form>
Try that. I may have left something out like a return true; at the end.
Hope that gives you something to start with...
Thessa