First, you shall no there is nothing impossible in this world ;-)
However in your case, to achieve
only that would be a big work, because this JavaScript alert is used in some other cases as well. Earlier I
explained the case in the related thread.
Basically, this alert is generated under form.js -> submitForm() function. As you may see there are many conditions which lead to the same result, and not all are related to users guest status. If you are powerful enough to re-code it to make the different result appear only for your private case, you have a go.