I'm not sure what you mean by "so I'll have to switch that to drip feeding from the indexer control box via RS232 and M codes"
Most indexers have a "program" loaded that has several moves or a single move that is repeated. The control would issue a command (M-code) to the indexer to execute the next line. Once the indexer is in-position it issues a FIN (complete)
signal back to the Fanuc and the Fanuc then continues on with the program. This operation type is why you can not interpolate (under normal setup / practices) with an indexer.
The indexer would not receive any type of drip feed.
So if RESET is pressed on the Fanuc. The program would stop and no further index commands would be sent to the indexer control. Now, if the indexer was executing an index, that move would most likely complete unless there was handshaking to interrupt it.