There is customizing which can help you call a workflow from DMS status changes, you don't need a BADI. go to trascation DC10 -> select your document type -> status -> details and you should see a workflow box, in it define the workflow you want started when the status is set.
Pay attention that the workflow activation will be done by by the DRAW event 'CHANGED' and DOKST as a receiving function module which will start your workflow you defined.
P.S
Using a user->approving user (dept head) tables will usually lead to high maintenance. for your agent determination try using data from the document if possible (lab/office for example) and workflow responsibility rules (transaction PFAC).