I just ran across the phrase "button enabledness" in a technical design, but that doesn't look right. Is it a valid word? Is there a better way to express "the state of whether or not something is enabled" (even if it doesn't derive from enabled)?
I think enabledness is a real word. Words like antidisestablismentarianism show that you can combine affixes in many valid ways; it's a fairly general mechanism, like combining clauses into sentences. The rule is that -ness can be appended to an adjective to form a word that refers to the state of an object that may or may not have that condition, and enabled is a valid adjective describing such a condition.
I think it seems cumbersome because the adjective is a gerund form of a verb, and -ness is more often used with simple adjectives, e.g. sadness. Off the top of my head I can't think of a simple adjective that means enabled, so that we could append the suffix to it. When new adjectives need to be coined, they often come from verbs like this, and you end up with unfamiliar combinations as you derive other forms from them.
If you want something that sounds more playful, you could use enableditude.
UI elements that are enabled are also sometimes referred to as active, so activeness could be another alternative.
I agree with Barmar's answer. It's just that the concept of a button being enabled or disabled is so far from most folks' concerns that enabledness is going to sound funny. By the same token, a logical equivalent, disabledness, sounds even worse.
Avoiding an unfamiliar construction at the cost of some brevity, you might try button enabled/disabled status.