Can I get master plan key from a branch's BuildContext?

[Ed Jackson]

I have written a plugin which stores some plan-specific information in its configuration, indexed by plan key.

I need to access the configuration from a CustomPreBuildQueuedAction. To do this, I need the plan key, which I've been getting like this from the BuildContext that's passed into init():

String planKey = buildContext.getParentBuildContext().getTypedPlanKey().getKey();

This works fine as long as I'm on the master branch. However, if I'm on a branch build of PRJ-PLAN, then the key will be something like PRJ-PLAN0 and the lookup will fail.

Is there any way to look up or derive the main plan's key from a branch build? (Lopping off a single digit if buildContext.isBranch() is a hack that will certainly break at some point...)