If you are only sending mails you might not need to use workflow at all but send the mail directly (using class cl_bcs and cl_bcs_document for example).
As Egor said, using a workflow responsibility rule does not require coding if it's used as part of the workflow, simply define the lab in the rule container and transfer it from the object to the rule. for a send mail or a direct send you can read the data using function module RH_GET_ACTORS
swc_set_element lt_container 'Lab' lv_lab.
call function 'RH_GET_ACTORS'
exporting
act_object = 'AC<rulenumber>'
tables
actor_container = lt_container.
actor_tab = lt_ac_agent.
exceptions …