If various Studs down here were to voluntarily limit the books of some of their sires & advertise their fees on that basis then that would be fine.
I do however have a very big problem with legislated book limits in this marketplace...especially by way of the fact that only a minority of foals bred every season in Australia will see a sale ring.
I very much doubt it's ever going to happen because the slab of off-season income that the Northern Hemisphere Farms/Syndicates receive from the Southern Hemisphere activities of their sires is money for Jam, they all know that.
Furthermore, while they will not come out and say as much publicly, because of the fact that over & above the $ paid for the Southern Hemisphere Rights nearly all of these high profile horses are standing on the ongoing basis of a % of Southern Hemisphere generated income being repatriated to the US Syndicates. As such they're more than happy for their sires to come down here and hit every mare that moves because it has absolutely no bearing whatsoever on the US Yearling Sale Market. They're on an earner to nothing.
Don't be fooled by the tut-tutting that goes on from time to time out of the US with regard to some shuttle sires hitting big Southern Hemisphere books. It is just window dressing for the US marketplace. Those crazy Southern Hemisphere Farms are at it again. Behind the scenes though, the syndicates are just like everyone else in business. They're all rubbing their hands together with glee and thanking the Lord above for their good-fortune.