[nightstrike2]

Thanks for the pointer. I seem to be on the right track with this:

local f = ItemTooltip["SetStoreItem"]
ItemTooltip["SetStoreItem"] = function(self, index)
    f(self, index)
    local types = { GetStoreEntryTypeInfo(index) }
    for i = 1, #types do
        if (types[i] == ITEMFILTERTYPE_FURNISHING) then
            self:AddLine("xxx")
            break
        end
    end
end

But now somehow I need to know whether the achievement text is already there. I used GetStoreItemLink on the entry and dumped it out to chat, and discovered that the item link itself doesn't contain the red achievement text in it. So, it gets added somewhere else. Do you know what adds that text, and how it gets added?