Thought some good samaritan would have contacted since you first posted this.
These pictures are great for reference. https://www.rib.net/forum/f36/need-t...380-84408.html
Are you sure you can't split the throttle? That would be obvious route. Where there's a will there's a way. Take it you have the code on what's left of the key to order a new one, or a spare?
Not sure if this will work, but worth a shot. Remove the throttle from console, now spray penetrating release into the lock barrel such as PlusGas, rather than WD40. Next get a junior hacksaw blade with the clip at the end removed. Lube it up too.
With the remaining key can you put the barrel into the off stage where you would normal extract the key and at the same time insert the hacksaw blade? Hopefully you can insert and there's enough of a barb on the blade to pull the remaining part of the key out. Failing that, give it a knock on a hard surface to see if it jolts the remaining key section to move so you can extract it with a jewellers' screwdriver.