Thanks for taking the time to comment. I am delighted to tell you there is a way you can do this in KashFlow presently!
1. Go to Settings > Email Templates
2. Select your invoice template
3. In the subject line Enter [INVNUM] followed by any text you want in the subject line
4. Save the template
5. You will see the invoice number has been inserted
If you need any further help with this, please contact support on firstname.lastname@example.org
Natasha - Product Manager