Making them ancient Frost Treants that have a "frozen death" is a good idea, and it fits with Hoar, since hoary means "white or gray with age; ancient", although I'm thinking you're also referencing Hoar
-Frost ""rime or white frost, the white particles formed by the freezing of the dew".
I still don't like "Hoartreants" as one word, so shall we call them "Hoar Treants" or "Hoar-Frost Treants"?.
That works for me. In that case, just take frost treant's ability scores (with no Con score, of course), and perhaps boost Cha a bit (as is common for undead templates)?
Sounds good. +6 Charisma, since we've been talking about giving them SLAs?
Hoar-Frost Treant: Str 25, Dex 8, Con --, Int 10, Wis 14, Cha 16
They're going to have a serious hit point drop (averaging 4 points per die from losing +6 Con bonus but gaining d12 HD). Should we give them some better DR or a HP-increasing SQ to compensate?