Does anyone know how to automatically subscribe to a document?

Once you see the power and usefulness of subscriptions, the shortcomings become very important.  Using default one user <> one object:

- Cannot easily set up many at once

- Cannot transfer from one user to another

- Cannot set up if objects don't yet exist but are expected to

So, it's worth investing some time and effort setting up:

- Subscriptions to Folders, then select object(s) in the Folder

- Make the subscriptions to a Group; separately add to /remove users from the group

Since it's all the time, every time before a specific Object exists maybe use Notification Rules?  I'll qualify by saying that I have not tested to confirm, but I was able to set a notification to a group, triggered by a Check In event of a specific sub-type of WTDocument within a specific Product/Library Context (could do at Org or Site level if you wanted).

What this won't give you that a Subscription would is control over content in the notification email... instructions in the message body etc.  I expect the Notification Message is generic enough that it would become noise if recipients did not know that something was required on their part

So if you absolutely need Subscription I would probably take the approach of either a workflow on the initial State of the Object to programmatically set the Subscription (which would require a unique life cycle for the Doc sub-type)or creating a custom action to do so for specific sub-types.