I'm not sure you'd want to feed a reactor from overflow. No way to increase flow if needed. Better to create manifold off return for GFO and carbon. Have those dump back into sump. You don't want to try to go directly back to return line because of back pressure issues. If you want to see the cleanest sump setup check out 'drummereef's 180g in-wall build' on Reef Central. There's an index of the thread on page 100! It should be subtitled 'When OCD meets Reef'