Unfortunately, that wouldn't be possible. The only details you can know about the spotter is the following:
enum irsdk_CarLeftRight
{
irsdk_LROff,
irsdk_LRClear, // no cars around us.
irsdk_LRCarLeft, // there is a car to our left.
irsdk_LRCarRight, // there is a car to our right.
irsdk_LRCarLeftRight, // there are cars on each side.
irsdk_LR2CarsLeft, // there are two cars to our left.
irsdk_LR2CarsRight // there are two cars to our right.
};
And detailed positional data simply isn't available through the live API either. The closest you get is how far along the track a car is, as a percentage of the total track length. I recall reading some time ago, though I don't remember where, that this is a deliberate attempt to curb any form of cheating. Live access to information like ride height, tire pressure, and tire temperature is likewise not available.
In the last race I saw from the iRacing streamer Dan Suzuki, he had some kind of visual spotter - I think it was from Kapps - where at least you saw the position of the car - how far ahead or behind - on the side.
Perhaps that‘s implemented with the percentage you mentioned.
Can confirm this is from Kapps, and I believe it was added recently. Not sure how he does it, but he streams on Twitch[0] and you can probably just ask him in chat if you catch him when he's live. He's live right now actually (7:15AM EST).