Default Branch Plan Variables

We use Bamboo Cloud. I'd like to have certain variables for branch plans set by default when the branch plan is detected/created.

The particular use case revolves around Salesforce metadata. For a "master" branch build it should do a deploy to the Salesforce org. For a branch build it should just verify that the branch is deployable (and not actually do the deploy).

I have an adequate solution already as part of the ant job:

<condition property="sfdc.checkonly" value="false" else="true">
<equals arg1="${bamboo.planRepository.branchName}" arg2="master"/>
</condition>

This is OK but it creates a dependency on bamboo, and on the name of the master branch ("master").

It would be cleaner if I could just set "sfdc.checkonly" to true for Branch plans. My script will have fewer dependencies and be locally verifiable.

Variables are already editable in Branch plans, so I think this would fit with current features.