As to whether a NHL designation has any effect or control over the preservation or limitations on changes that can be made to a property designated as a NHL, one should read the fine print in the NHL website.
Basically a NHL designation does not change an owner's ability to make changes though, unless of course that owner is trying to avail himself of local or federal monies to maintain that NHL in some way.
Certainly there may come a time or point where an owner of a NHL designated property may make so many changes that the NHL designation may be reviewed or withdrawn.
When it comes to ideas about preservation, I think some of you may be confusing a NHL designation with a "conservation easement".
It's certainly possible to place a "conservation easement" on a property that even the Federal Government (although generally local governments) may consider historically interesting or perhaps beautiful enough to be worth perserving 'for the public good'.
But the "easement" is an agreement or covenant with that government, generally through a conservation organization (which I call an "aegis"), that requires certain limitations on change or alteration by an owner.
In almost all cases a local "conservation organization" (basically approved or franchised by the local or Federal government) writes the "convenant" with the owner specifying in detail what can and cannot be altered. And that conservation organization is responisible for monitoring the "easement" for that government to be sure it's being adhered to by the owner.
The interesting thing about conservation easements, which generally involve the preservation of land as open space (although they can involve the preservation of things like historic buildings) is that they come with rather significant tax deductions to the owner--and those deductions generally involve Federal income tax, although it can also involve local tax abatement of one kind or another.
If the owner breaks or abridges the "easement" obviously he can be responsible for repayment of tax abatements, deductions and probably penalties.
But the basis of Federal conservation easements (through local conservation organizations) of this kind whether on buildings or land is it must have or do some good in a public sense, or obviously the governments cannot support the use of Federal or local monies (even in the form of personal tax deductions) to support the conservation or preservation.
This could be done for an historic golf course but it's a little hard for private clubs to take the tax deductions and at the moment there's no conservation organization I know of who would be willing to monitor the preservation of a golf course, like a Merion or Oakmont.
And secondly, would an Oakmont or Merion ever want a conservation organization to monitor the preservation of their golf course? But sometime money talks, even if in the form of a tax abatement or deduction.
At the moment though, I'm certain there's no actual "conservation easement" on any golf course in America except for the fact that the land underneath a golf course may be restricted as to it potential buildability if the course was to cease to exist. And I only know of one golf course with an arrangement like that.