Thanks!
It's a formal process now. Top 10 alts can ride practice and if you want checked in as alt you have to ride practice. So I rode 45 and 50 practice Monday.
On the results board by the finish line they write down Alt names as they get in during the day Monday.
Then for moto 1, you gear up and go to "alternate island" which is a canopy over by mxsports office. They come out and check you in as present and know your alt order.
You all sit there BSing waiting to see what happens. If someone doesn't show to the gate, they radio up and say send down an alt and they walk out and tell first alt to head down!