Maybe this is a change or just a fluke - but we were able to purchase
Genie+ on 9/19 with just a party ticket (no Magic Kingdom park reservation or park hopper). We selected MK as our park in the My Day tab and it let us purchase and book.
We didn't arrive with stacked LLs - since most return times were almost immediate, we just churned through as many as we could between 4-5:30pm. Haunted Mansion, Big Thunder, Splash, and Under the Sea (we absolutely did not need a LL for that last one). We were also able to pre-purchase a LL+ for Seven Dwarfs. Standby waits were posted around 15-30 min during this period - so did we need Genie+? No. But it let us knock out some rides quickly and then focus on the party after 7pm.