Programatically enable/disable a build step

I am implenting a controller to create builds according to a fairly simple set of rules.  I have created a largish template with multiple steps and now want to enable/disable builds steps on the fly

In my controller I am using SBuildType to access the runner collection via SBuildType.getBuildRunners();  So far so good.

The problem is that thre are no methods to enable/disable the various runners.  I had a peek at the properties collection and I have three of them:
  use.custom.script
  script.content
  teamcity.step.mode

teamcity.step.mode looked like a candidate but I can only find the following property name:

BUILD_STEP_EXECUTION_MODE

which appears to only have the following allowable values:

BUILD_STEP_EXECUTION_MODE_DEFAULT

BUILD_STEP_EXECUTION_MODE_EVEN_IF_FAILED

BUILD_STEP_EXECUTION_MODE_ALWAYS