How can I dynamically set the questions.yaml default value from a global configmap?
You can’t, that is not a thing that exists.
Notwithstanding that it does not currently exist as a feature, you can always raise a request for it, explaining your use case and why it would be of benefit. Indeed, I would encourage you to do that here also to at least test the water as to whether others using Rancher would find that beneficial and add their support to your PR/feature request ? As no doubt you are aware, Rancher, much like other maintainers will tend to focus on new/change requests where there is greater demand.