If you add an external worker to the default worker pool then Octopus will by default use that worker instead of the built-in worker. You could also add some external workers to a different worker pool and change your terraform steps to execute using that worker pool instead.
Progress has been made on the binding for worker pools along with scoping, however it’s currently on our backlog as we have some other more urgent priorities. I would love to provide an exact time frame, but would be unable to do so at this point in time.
We do have another feature called dynamic workers which isn’t too far off, that will be available to hosted customers. There could be a number of directions we could possibly take this, so would definitely be keen to hear if you have any ideas of what features you would expect from something like this and what would be useful features for you to have.