Oh I think we understand the behavior of the light pretty well, it's well documented in the AOH and Systems Handbook. There is two sources for the light: up-data link (UDL) abort request and pad abort request/range safety S-IVB destruction arming. Each of these having redundant signals, power and light bulbs.
The UDL abort request is powered by the EDS, but doesn't otherwise use any EDS circuits or logic. I believe there are three sources that would send this abort request. Switches on the consoles of the launch director, the flight director and the FIDO. I've heard all of them being tested in the Apollo 11 MOCR audio during the EDS test.
The second signal comes from the IU, either from a "GSE pad abort request" signal, which is what also would have been tested during the EDS test, or it comes from a signal from the S-IVB, caused by the range safety officer arming the flight termination system.
In NASSP most of the relay logic for all this is implemented, but there is no flight termination system yet. The UDL abort request can be set with the telemetry client and the GSE abort request can be set in the mobile launcher code.