Funny that this stillicide conversation should arise again at the same time I am teaching Coleridge's "Frost at Midnight" to my Intro to Poetry classes. That coincidence drew me to the final passage:

whether the eave-drops fall

Heard only in the trances of the blast,

Or whether the secret ministry of cold

Shall hang them up in silent icicles,

Quietly shining to the quiet moon--

Like those, my babe, which, ere tomorrow's warmth

Have capped their sharp keen points with pendulous drops,

Will catch thy little soul, then make thee shout

And stretch and flutter from thy mother's arms

As thou wouldst fly for very eagerness.

eave-drops] Kinbote glosses stillicide as "eavesdrop."

icicles] the frozen stillicide

shining to the quiet moon] a mirror image, the icicles stealing (and giving back) the pale fire of the moon

I think "Pale Fire" continues and perhaps even (via this passage) acknowledges a tradition of father-to-sleeping/dead-daughter poems that runs through Ben Jonson's "On My First Daughter," Coleridge's "Frost at Midnight," and Yeats's "A Prayer For My Daughter." (Weldon Kees's wonderful, horrifying sonnet "For My Daughter" also plays with this tradition, but I doubt VN knew that poem). That's not to say that Coleridge's unstated stillicides replace Hardy's stated ones, just that there may be a secondary resonance here--just as "pale fire" finds a secondary association in Hamlet.