The auto tour routes go north from Genoa NV on Jacks Valley Road (I think this was my bad for just saying SR 206).
No, it's my bad for looking and not seeing the signs at the north end of Jacks Valley Road, seeing 'Mormon Station' and treating it as a sign marking the nearby attraction rather than the route, and then seeing that the Mormon Station is signed from US50 taking the current route of OR206, rather than Jacks Valley Road. I'll fix it this morning.
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.
I only found the one sign for the Tahoe route, but that doesn't mean there aren't more - if I found more, I'd include it (like with the Lincoln Highway).
Looking now, I found a couple of signs in California, but they are of the annoying 'Heh, the next mile is part of the original Pony Express route' type rather than the Auto Tour type that are designed to join up (if the signs are there, at least!).