I would like to be able to have a variable that contains the plan key at build time (projectKey-planKey)

Documented here:
https://confluence.atlassian.com/display/BAMBOO/Bamboo+variables

We have this during a release plan, but not during a build plan. I would like to be able to tell thirdparty tools what the key is so that when they later call back into the rest api to initiate the next stage of the plan, I have the plan key which is needed for the rest call.

currently I have to user either:
bamboo.buildKey and chop off the "-JOBx"
or
construct it using calls to http://my.bamboo/rest/api/latest/plan/ etc.