Yes, it has been tried and tried countless times. The history citations you alude to (travelors casting off belongings to lighten their load) are nothing new. And the routes they took are also nothing new. Many books have been written detailing the exact routes. Even in stretches where modern roads do not follow the original routes, the old routes are "no secret"
So even since the 1930s, when auto-travel made it popular to get to more remote places, collecting nick-nacks, bottles, collectibles, etc... was already in full swing. For example, in the late 1950s/early 1960s, a lot of west coast "ghost towns" were already being stripped of visual artifacts, thus leading to the preservation efforts of those that were remote enough to not already have been pillaged (like Bodie, in the early 1960s). Yeah, those earlier times would be top-of-the-ground visable things, but the point is, it's been long-ago thought of.
There is a particular stretch of desert called "the 40 mile desert", where a lot of what you're aluding to went on (lightening loads, deaths, etc...), and people for the last 30 yrs have detected it ad-nauseum. A few coins have been found. But I think the "lightening of loads" would primarily, as you say, been things that were heaviest, not coins. Your best bet for coin hunting would be where they camped for the night, and be regular fumble-finger losses. In that sense, the coin-hunting potential would be the same as any other campground setting, whether for CA travel or otherwise.
The club in San Jose, for example, organized a group hunt to work the 40-mile desert. They mostly find caste-iron type stuff. I believe a coin or two was found on those expeditions, but no more so than a person would be served in hunting any other period place, or period campground.
There is a good book on the subject called "The California Trail". I don't have the author's name in front of me, but it was written in the 1940s, if I recall. At that time, the author could still (and did during the 1930s, for instance) interview older people who could actually recall the travel as young kids. The "castoffs" you refer to also happened at the Eastern end of the trail (once they got to the plains states). If I were gambling on a purposefully stashed cache of coins (as if someone would choose silver and gold to lighten their load, verses iron implements), I would hunt campsites. Quite often, the travelers used and re-used the same sites for camping (typically along rivers, or any watering hole site).
