Hi Lee,
You can do something similar in KashFlow at the moment;
If you select the customers you wish to email using the checkboxes on the left hand side within the customer screen, you can then select an action at the bottom of the screen labelled email statements.
This will then let you select the date period you need and also let you exclude paid invoices. It will then generate and send those emails for you.
Does that solve the problem for you?