If it is a pure sink then the SPA will be the original one you load in the initialization EDID table along with the SPA_B that you configure.
If it is a repeater then the EDID will be derived from the sink.
Check out the 7612 hardware guide section "Structure of internal E-Edid for port A", page 33 how to set up the SPA information, specifically SPA_LOCATION and SPA_PORT_B registers. SPA_LOCATION points to where in the EDID block the SPA_PORT_B values are inserted into as the EDID is read by the source.