Hi Alan,
To do this set up the following:
V3 keyed out subject
V2 keyed out subject
V1 background
put your Chroma key on V3 and select the “show alpha” checkbox.
apply ScanLines to v3 by alt+dropping it and then select “apply to title/key” checkbox in the effect editor. This will merge your matte (v3) and fill (v2) and comp it over your background layer on V1.
Best,
Todd