Subliminalness? Subliminality? My husband wants to know 
There’s not one, tell him to create one.
Sublime? A thing that sublimes/sublimates is called a sublimer.
I will tell him.
I’ve seen “subliminal” used as a noun, usually plural. From Deathwatch: “Your subliminals aren’t working right. I can hear them.”
“Subliminal message” is probably the most common way to refer to the concept.
“Stare into the subliminal, for as long as you can.” -They Might Be Giants
Sublimation?
subliminality?
Would I be booed terribly for observing that this thread is rapidly moving from the subliminal towards the ridiculous?
There is precedent with liminality. I wouldn’t be surprised to find citations for “subliminality” for what you seem to be referring to - the quality of being subliminal.
Sublimity?