I don't believe there is a server object tied to an IFrame. Not sure if you
would be able to add an id and runat=server attribute to the IFrame to allow
you to access it on the server but it might me possible. Not sure what
control you would need to associate it with in the server code.
If JavasScript is an option then this should work:
Right before you submit the form, use JavaScript to put the innerHTML into
another server control that is accessible on the server such as a textbox or
maybe a Placeholder or Panel. An standard input type="hidden" control with
Runat=server control with matching server code control declaration
HtmlControls.HtmlInputHidden txtTB; should allow you to access the
transfered innetHTML from the hidden textbox on the server.
Hope this helps!
Andy