I had a client who needed custom fields on subtasks and we did the following:
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Create a new project called "Custom Fields [not in use]
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Add the desired custom field to this project.
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When a subtask is created, use the “Add to project” option in the … menu to add the subtask to this project.
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Now the subtasks inherit the custom field from this project.
It’s not ideal, but get’s the job done.