Hmmm, on the one hand you say :
" ...I haven't actually looked up the laws pertaining to sluicing...."
But on the other hand you say:
".... any time anything is mentioned about the legality of prospecting streams/ rivers in California, the only mention of doing it is with a pan. It expressly forbids mechanized dredging/hibanking and as far as I can tell makes no mention of "natural sluicing".
So do you see how the 2nd statement appears that you HAVE IN FACT looked into the laws. Lest how else would you have been appraised of those other things you "can't do" ?
You say :
"...I don't want to run into an overzealous lawman who decides to confiscate my gear and hands me a summons just because his interpretation of a vague law is not the same as my interpretation...."
Couple of observations on that : If there had ever been a case of anyone "getting their gear confiscated " for this type sluicing, then YOU CAN BET that there would be links passed around about it. So have you key-word searched it ? I'll bet you dollars to donuts that you are not going to find any such incident. And that no one can ever recall such a thing. You are even saying that it's a "vague law" and "up to interpretion". As if to acknowledge that there is NOT any "black and white disallowance". So where does this fear of confiscations arise ?
Whenever this fear of "confiscations" (and jail, and tickets, etc...) arises in the discussion of someone asking about innocuous benign md'ing locations and types, I always ask them if they have ANY examples of jail, confiscations, etc... And all you ever hear is the sound of crickets.

And if they CAN cite any example of confiscations etc..., it will always be for someone night-sneaking obvious historic sensitive monuments. Or someone who is being obnoxious and can't take a warning. Or someone doing a site that, with an ounce of common sense, could have easily found an explicit law to the effect (which you're saying is not the case here).
So why do we keep fearing "confiscations" so much ? Easy: It's the psychology of shark attacks: No matter how rare, no matter how many thousands of people swim every day without incident, yet if a SINGLE attack occurs somewhere in the world, guess what will appear on the next day's headlines ? And then guess what everyone will fear the next time they go swimming ? So too is it with md'ing.
Thus people tend to want express allowances. Ie.: a "yes you can" is much more smoothing and comforting, eh ? So they go asking forest service people "Can I?" type questions (as if they're asking permissions, and as if our hobby were somehow harmful or evil that it needed sanctions and approvals). And thus, presto: Another law or rule safe-answer is born to "address the pressing issue". And we end up being our own worst-enemies
So again I say : If there is no law or rule that says you can't, then presto, it's not forbidden or disallowed.