the el camino real was said to have evolved into the modern Hwy. 101. That's only true to a degree. Ie.: give or take several miles, in various locations. It's not the exact path of current Hwy. 101. Just approximate
And the places you'd want to find and hit, are the stop spots. Not just "random along the route".
Get a book called "Historic spots in California". There's some stuff about adobe sites down in that area of CA. But as for getting someone here to delve out a spot where they know old coins are still possible, is probably not gonna happen. Old coins are not a renewing resource. Therefore, the hard-core guys, who have found an adobe site, stage stop, etc.... that is producing, will tend to keep mum, and work it with their buddies.
Kind of like surfers who don't tell others about a good coastal surf break spot. 'cuz the moment other surfers arrive to crowd it out, it's no longer fun. Or a trout fisherman who discovers a hole on the creek or river, that has trophy size trout, etc...
But there's "Spanish" (mission era) stuff down in that part of CA to be found. We've gotten some reales, buttons, etc.... down that direction.