Create a plugin for AcyMailing that would do the following:
1) Grab a query string from the URL. This query string would contain a list of email addresses, separated by commas.
2) Display the list of emails to be sent to in a textarea.
3) Allow the user to send the email to these email addresses.
The attached image shows how this should look and how I hope it will work. With this plugin, it would no longer be necessary to create an AcyMailing list before sending emails.
In order to test as you develop, please download the free AcyMailing plugin and Joomla on your own server.