To me, you are presenting one of the few genuinely interesting arguments against the existence of an objective morality. Most people who chose to argue against the existence do so on the grounds that opinions of what is moral can vary widely from one person to the next.
But that is no more an interesting argument than it would be an interesting argument to suggest that 2 + 2 has no objective sum because there are numerous conflicting opinions among first grade arithmetic students as to what the sum is.
Your argument, on the other hand, if I understand it correctly, might be boiled down to: (1) If morality existed objectively, then it would be in some way empirical, (2) it cannot be sensed empirically, therefore it does not exist objectively.
To me, that's a vastly more interesting argument than the much more common argument -- and fallacious -- argument that morality cannot exist objectively because people have different opinions of what it is.
I'm not much into nihilism--concluding that something doesn't exist because we cannot sense it. I see that as a fallacy.
There are many things about objective reality that we can only sense with the help of technology. For instance, CERN has been expanding the empirical data available to us about objective reality. Just because we can't sense something, doesn't mean it doesn't objectively exist. It just means we haven't found it yet.
The range of sensory accuity can vary greatly between individuals, as well as the ability to express sensory nuances consciously. One person may be color blind in regards to the color "red," another may sense red fine, but only has one name, "red," for a general range of colors, or a few names for this spectrum, whereas an artist may have hundreds of names for various shades of red. This might also be the case with individual variences regarding morality, if it is something sensed.
There is also evidence that color sensing might also have a cultural factor to it. Some cultures have difficulty discerning "blue" from "green." Such factors might also play a role in morality.
There is also evidence that our innate sensory range is more sensitive than our conscious minds can process. We might be able to sense a wider range of things than we realize--but just don't know what to do with the data. (People who can successfully douse for water being one example.) Morality might fit into this empirical category--we can sense it, but cannot bring it into consciousness fully.
Human beings generally recognize that there are instances where one might be morally impaired--leniency is generally extended to the insane acrossed a wide range of cultures. Children are also granted leniency, in that "they don't know any better."
So, what we are faced with is this:
- Is morality a delusion (a subjective product mistaken for something objective?) Is there a kernal of something there that is covered by delusion?
- Is morality useful? What is its range of its usefulness? Does it become unuseful at a point? (Such as stoning someone on the spot.) Does the area of usefulness correlate with "real" and the areas of unusefulness correlate with "delusion?" (This is an empirical test in a way.)