I already looked at
but that didn’t help a lot, since the example it gives isn't very similar to my case. What I ended up doing was putting these lines into the library Jamfile:
project workflow_mgr :
usage-requirements <implicit-dependency>/workflow_mgr//Mock_workflow_mgr_core.hpp ;
mockgen workflow_mgr_core ;
The mockgen rule calls the make rule to generate header Mock_workflow_mgr_core.hpp from workflow_mgr_core.mockgen.