It would be nice if the Jenkinsfile could set/override values on parameters (i.e. assign to params.*).

Why you might ask. Frankly, my use-case is to work around JENKINS-53492, namely that there is no way (i.e. DSL step, etc.) in a Jenkinsfile to set a priority in the Priority Sorter plugin for the currently running job.

So my hack-around is to use the Priority Sorter plugin's ability to get the priority from a parameter. This works fine for use cases of triggering a build with Build with Parameters however it doesn't allow one to code a priority into one's job by way of Jenkinsfile.

There are very likely other use-cases for being able to have a job set it's own parameters.  Or maybe there isn't.