I know there is a free version of draw.io for Confluence Cloud, but if an Atlassian partner, for example, wanted to be licensed is it possible to get the pay-for version without cost?
Yes, Atlassian partners and charities qualify for free licenses to the Enterprise draw.io offering.
You can find details of how to request and setup your license in this thread.
Note that only the functionality elements of the Enterprise product apply. You do not get the SLA or the extended support parts of the Enteprise offering.
Fore more general info on draw.io for Confluence or Jira, please refer to our draw.io webpage.
And some great blogposts! Blog - draw.io