Is there any way to set an APM perflow variable in version 12?

I'm using an APM per-request policy in version 12. It would help me a lot to be able to set a perflow variable.

 

My first thought was that perhaps I could do this with an iRule, but the [ACCESS::perflow] command in version 12 only allows you to get perflow variables, not write to them (that's apparently a version 13 feature). So iRules seem to be out of the question.

 

My next thought was to use a variable assign object to do it, but those only seem to exist in the regular/main APM policy, not in the APM per-request policy.

 

Is there any way at all in version 12 to set one of these variables?