Set Plan Variables through REST API

Its possible to set Global Variables through the REST API, and to set Plan Variables when executing a build but I can't find a way to permanently set a Plan Variable.

Rationale is I can create branch plan's, set to my VCS branch through REST API, I can enable, favourite and add labels, but the most important part is to set the Plan Variables which override the parent (or global) variables for this branch.

Referencing https://answers.atlassian.com/questions/35774012/how-to-update-plan-or-global-variable-using-rest-api and https://jira.atlassian.com/browse/BAM-9344 but can't see if this is or isnt a dark feature?

If it is - can I have an example of POST setting a planvariable?

If it isn't - can this be added asap to make the REST API useful for creation of plan branches?