Description

This post-function updates text custom field (of your choice) in all subtasks with given value (or null).

Configuration

in Administration-> Issues-> Workflows edit a workflow that is used for Test Plans (not for Test Cases if vary)
edit Transition, which is to have this post-function (if many, repeat for every of desired)
add post-function TestFLO - Update Subtasks CustomField Function
choose a text type custom field that will be updated during the transition and set its new value. If value field empty, the field will be cleared.