You could potentially set something up depending on what is being watched.  If you have a specific entry in the repository that you are watching for the metadata change then you could pass the approval in a variable.

Here is a basic idea to get you started.  On the form, you will need to have a "Start Workflow" activity.  Make sure you set the activity to wait until the workflow is finished before proceeding.  You might need to store a copy of the form first in the repository if you need an entry to watch in the workflow.

The workflow itself can do whatever you need it to do such as name or file the document somewhere.  To get it to send the approval back, you would need to watch a specific entry, put in a wait condition and wait until the metadata field changes on that entry.  After it changes, you can send the value back to the form.

Once it goes back to the form, you can have a decision point in there that will move to the next step of the form process based on the value that is fed back to the form.  For example, if it returns "Denied" you could end the form process, or if the workflow returns "Approved" you could have the form assign a task or whatever you need it to do.

Hope this helps point you in the right direction.