A relay should do the trick, 10ms delay from the machanism shouldnt mess things up too much, if you want you could replace the shift led with the led section of an optocoupler, and use the phototranny part of the opto to drive a transistor whch switches the relay.
You say the system is sequential, most systems switch modes from fully sequential to 2 at once then all 4 at once (batch fire) as rpm increases towards max, though this shouldn't be a problem for your application.
Also later systems monitor the current drawn by the injectors, disconnecting them may put the ecu into error mode, switching to a load resistor might get around this if you switch it quick enough.
Oh yes, welcome to the forum.