Is there anyway to store additional data in the state file outside of creating a resource? To give more detail, I've got some plain C# code that calls a rest api as part of a Pulumi project ( there's no Pulumi resource for it) this works fine, but currently it gets called every time I run Pulumi up. Ideally I'd like to only run this on the first run the deployment, so if I could store the value in the state file and check for that existing this might deal with that.

this might be what you're looking for https://www.pulumi.com/docs/intro/concepts/resources/dynamic-providers/

unfortuantely dynamic providers aren't supported in .net, else yes that would do the job.