Is it possible to write to a stack Config from code? Looks like it only has Require/Get and not Set?

Not from within your Pulumi program which sets up the infrastructure. But the Automation API, as a way to programmaticaly create stacks can also set config values: https://www.pulumi.com/docs/guides/automation-api/getting-started-automation-api/#configure-your-provider-plugins

Ah, nice - that's exactly what I was looking for (automating stack building). Thanks a lot!