Kam, can you see through all this verbage? This is clearly written with all the archaeological buzz words and phrases. This "Paul Lusignan" guy is probably an archaeologist himself. No problem in that, as we all know that they, as a group, would prefer that all md'rs dry up and blow away. So if you ask enough of those type mindset folks, you will get all your answers colored with their bias, hopes, etc...
Notice a few things: First, he admits that if the plaque is on private property, the owner can do whatever he pleases with it (grant access, allow md'ing, etc...). So far so good, as this Paul guy can't deny that legal truth! But then he drones on and on with lines that are meant to discourage you (as all good archies would do

). But read carefully: All the "antiquity statutes" "laws" "permits" and other daunting vocabulary he throws out apply to PUBLIC land, not private land, which was the root of your question. An example of that is this quote:
"
Such statutes protect archeological sites by requiring a permit to excavate sites on public land" (emphasis mine)
Doh! even though your question was about private land, he can't help but remind you the daunting fed and state land stuff, just to insinuate that "oh NO! you better watch out!" It's almost comical. And then, as if he
just then realizes that none of this applies to you, he adds this "you better be scared" line:
"
The antiquities statutes of some states even cover private lands " (emphasis mine)
This might harken to some states attempts to do that (Kentucky? others?), but even THOSE usually refer to "archaeologically significant sites" within private land. Not ALL private land. Or to the extent it may be private land in general, it's only as it pertains to indian things, or graveyards, etc... And heck, to the extent that someone could morph your wheat penny or IH from private land to be within some morphing of words, do you really think anyone monitors these things? I mean, think of it: If what he said in the last quote had
any stretch of truth to it (in the way he means to interpret to apply to detectors), then that would mean that, supposedly, there are states, right now, that don't allow detecting on private property. And that's absolute garbage! There is NONE of the 50 states that don't allow homeowners and private property owners from detecting, or allowing detecting. Even the most archie-riddled states like Oregon (where web links spread about a bottle digger who got harrassed) still have oodles of md'rs that go un-bothered. So Paul's words in this last quote are just B.S. He hopes to deter all md'rs, as any good archie would do, to feel good about himself.
Finally, he appears to be well-versed in the md'rs come-back lines, when he refers to what you MAY be thinking (you evil person you!

) when he says:
"
Even if the thinking goes that "If I hadn't uncovered the item, no one would even have known it was there," this
doesn't recognize the possibility that future researchers might not uncover the feature at some later point and be able to piece together an important aspect of our shared history "
Wow, I must hand it to him, he is well rehearsed in our "better on my mantle, than buried unknown for the next million years" philosphy. The problem with his "some later point" drone, is that it never happens! There is simply no government resources, time, money, interest, etc... to go after all the stuff in the ground. Even in places they try, they merely make a few 4 X 4 pits, every other year or so, at best. It would take a million years *just* to cover a few national parks, campgrounds, battle sites, yards, etc... It's simply not going to happen. His same stupid logic failed when they came after Mel Fisher, telling him his goodies belonged to the state of Florida. When the FL lawyers/archies tried to say that Mel wasn't recovering it properly, and that is should have been left to the state to do it, Mel's lawyers succesfully argued that REALISTICALLY, no state or the fed. would ever allocate tax-payer money to fund risky treasure hunts. It's the private sector that does it, risks it, finds it, preserves it, etc... So the state had to drop their "someday future archies will discover or dig this" bunk.
You can go hunt at any plaque site, to your heart's content, as long as it's on private property, and the owner says "go for it".