[Phanx]

This should work:

local DESTINATION = UIErrorsFrame

local ONLINE = ERR_FRIEND_ONLINE_SS:gsub("%%s", "(.+)"):gsub("[%[%]]", "%%%1")
local OFFLINE = ERR_FRIEND_OFFLINE_S:gsub("%%s", "(.+)")

local f = CreateFrame("Frame")
f:RegisterEvent("CHAT_MSG_SYSTEM")
f:SetScript("OnEvent", function(self, event, text)
	if text:match(ONLINE) then
		DESTINATION:AddMessage(text:gsub("%[(.-)%]", "%1"), 0.3, 1, 0.3)
	elseif text:match(OFFLINE) then
		DESTINATION:AddMessage(text, .6, .6, .6)
	end
end)

Change the first line if you want to send the messages to some other chat frame, eg. ChatFrame3

Use Vlad's addon creator tool to turn the code into an addon.