Open the html file up in an editor, and enter your own information in the form fields (nameservers, plan features, etc). Then put them both in a secure folder, complete the form and click Send. Try it sent to yourself as the recipient to see the results.
We have added to the functionality of this now. It's backed up with a mySQL database and has the capability to resend Welcome Emails upon request to the existing email address, or to update the clients' email address at the same time.
We use it on a non publicly accessible server, so our own is safe in that regard. What damage could it cause though? Someone could contact our clients and market their hosting services to them? No damage could come of that.
If you resend a welcome email, and change the email address, it updates that. If anything else has changed, then yes you'd need to modify the record directly (or just send a new email and add another record).