A late answer, but nevertheless:
>Although the ability to speak was a defining characteristic of Huorns
>in early drafts (VIII 47-55), and this ability is still mentioned in
>the final text of LR (LR 551), the ability to speak is hardly relevant
>in the narrative context of the final text. In the final text, Huorns
>are defined by their ambiguity (second only perhaps to their ability
>to uproot and travel). They move in shadow, vaguely heard or sensed
>but not seen (no one actually sees them move, except perhaps the Ents:
>LR 551, 539; Merry says "they seem to be able to wrap themselves
>in shadow" [LR 551]).
That very paragraph actually contains a clear explanation of the name:
'They still have voices, and can speak with the Ents that is why
they are called Huorns, Treebeard says'
>One potential obstacle I see is that _Huorn_
>is a Sindarin word (per Jim Allen 1978) and _huine_ is Quenya (S 358)
Yes, that'a point. Original aspirated PH becomes _h_ before _u_ in
Quenya only. PHUY- yields Q. _Fui_, _Hui_ 'Night' as well as _fuine_,
_huine_ 'deep shadow', but N. _fuin_ only (V:382).
>If the mixing of languages in this instance is permissible, I wonder
>whether "ui" treated as a dipthong might be a problem?
Oh yes, it is, there is no known mechanism to simplify a diphthong
like that. Examples of hybrid Quenya/Sindarin words do exist, e.g. Q.
_Ondohir_ which substitutes _-hir_ 'lord' from S. _hîr_, pure Quenya
would be _Ondoher_ (XII:210). But also given _#hu-_ instead of _*hui_
the possibility of a derivation from PHUY- just doesn't appear likely
by Occam's razor.
Still, your guess was not unreasonable. Tolkien apparently considered
an element _hô_ 'spirit, shadow' (PE17:86) > *S. _hû_ as a possibility
among others, although the translation is not very readable. Look here
for a quick discussion (use the browser search to get to _Huorns_):
But one should note that Tolkien wrote down almost every possible
retrospective interpretation for a lot of LR names and it's seldom
clear in favour of which possibility he decided (if he did at all). In
any case, earlier variants are a helpful means of analysis, often
showing the original intention for the meaning of a name. In this case
it's clearly 'talking trees'.