The auto tour routes go north from Genoa NV on Jacks Valley Road (I think this was my bad for just saying SR 206).
Correct, that goes up Jacks Valley Road, which
was historically SR 206. 206 was moved onto SR 758 east of Genoa in the 90s, with the former alignment north of Genoa relinquished. I can make the change if it hasn't been done already.
If we want to be really technical, the Pony Express Trail branches, with one branch following modern US 50 through the Tahoe basin. This branch to the lake has several signs on both sides of the state line and there were multiple Pony Express stations along this other route. In CA, it generally follows US 50 to Folsom. The southern branch, currently mapped, follows CA 88 and CA 89 to rejoin. When I get home from my current trip, I can do some more research and map this out. It is definitely signed in California, though. Few of these other signed routes would add significant mileage to the HB, as they tend to be signed along existing routes.
There are also "Pony Express Auto Tour Route" signs on the CA 24 expressway in Contra Costa County, which I confirmed today from BART. Yes, the Pony Express usually ended in Folsom or Sacramento, but it continued overland to Oakland about 20 times, and this is acknowledged.