Select custom assignee fields in automation action

Hi! 

I came across an interesting use case today that I'd like to submit as product feedback for automations. I'm sure it can apply to other available automation actions, but today's example/use case has to do with the auto-commenting (to @mention users). 

It would be awesome if we could select a certain Assignee Custom Field to target as an automated action for dropping comments/mentions.

Right now, we have the following preset options in the "Add comment or mention" action of any automation rule:

Followers, assignees, author, and approvers can be selected and will notify according to the option selected and whoever is designated as any of those roles on the work item. We can add specific users or groups, but that won't dynamically change. 

What's missing here is an option to tag whoever is assigned to a certain Assignee Custom Field. 

For example, we have a request form that asks for various contacts, one being the Marketing Campaign Owner (not always the author/person who is making the request). We want to be able to leverage a comment with instruction to whoever is added to this particular custom Assignee field on how to proceed when a request comes in.

Hope this makes sense and that other people can see the value in this!