Some (all?) AIS units can obtain their own GPS fix.  Use one that doesn't connect to the rest of the network.  If it doesn't have built-in GPS then wire up a separate system from a GPS puck to the AIS unit.  That'd let your boat squawk an AIS position.

But if this is open-ocean, then what are you expecting AIS to provide?  Collision avoidance by the yacht itself?  How is it going to show that info to the captain?  Apparently some satellite coverage exists for picking up AIS, not sure if that includes recreational vessel transceivers or not.

I believe some VHF radios with AIS receivers have a screen on them and can show some "some" amount of AIS data.  But to hide the GPS coordinates?

Then there's MOB or mayday conditions, how is that going to be handled?

Perhaps some sort of locked cabinet with something like a wire tamper seal could be used.  Or a cover over a chart plotter than blocks off it's use, but allows it to remain functioning/available for emergencies.  I'm guessing you could configure an NMEA bridge to control what PGNs get passed, so any 4" style displays wouldn't be capable of getting an GPS data. 

https://www.yachtd.com/products/nmea_bridge.html

But you'd need to have a very thorough understanding of the boat's network AND all of the PGN data that's /needed/ by the various devices.  I'd imagine a bit of changes to the wiring would be necessary to more fully isolate the data, but that also brings along other network issues regarding power and such.

@wkearney99 The boat will be set up without a chartploter. The point is to sail around the world 
AIS are required to have a dedicated GPS antenna. What is at issue here is not keeping the GPS location from the AIS but keeping the AIS from displaying the GPS data on a screen that the competitors could use to then navigate from. The rules say they can't use GPS at all. 

In the end we determined the race officials expect him to transmit and not be able to see it because they didn't have AIS in 1968. 

Some AIS units have their own GPS detection built-in (Furuno's FA-50 which I have on my boat, does this).  It requires having a VHF antenna set up for it, separate from any onboard VHF transceiver (or using a splitter).  Once configured it'll squawk AIS with nothing more than DC power to it.  Doesn't "have to" display the data anywhere.  

But how certain are you that the places these boats will be going will be able to be reliably detected using AIS?  Because that's an important factor.  I'm no expert on open ocean detection of AIS, but it was originally intended as in-reach of shore or vessel-to-vessel, not out in the middle of an ocean.  I've read a few things that indicate there are satellites that can pick up AIS, but I wonder what requirements exist for the on-boat equipment to make that practical?  

Otherwise you'd be looking at inReach or other systems to handle location and bi-directional text communication.   Do the rules exclude life-saving devices like EPIRB or MOB systems?

Because it's one thing to required to ignore/not use gear during race conditions, it's another to be willfully negligent ignoring useful safety options.

Something like a vesper xb6000 has a green light that stays lit if all functions are normal (transmitting and receiving OK, antenna swr normal)

You can also turn on/off what happens with NMEA2000 as well:

https://support.vespermarine.com/hc/en-us/articles/360001774335-XB-6000-Class-B-AIS-Transponder-Owner-s-Guide#h_01EG2FXYP1NEK2VWQ7SE4P596Y