harvest include non-billable tasks on invoices

So i wrote to Harvest support:

We’d like to be able to include non-billable tasks on invoices

Is there a hidden way to include non-billable tasks on Harvest invoices? We want to provide a full record of work for clients who are receiving some kinds of tasks at a 100% discount, while not inflating our ‘billable’ hours. If there isn’t a way yet… could you make one? Thank you!