Upon further reading in the default UI code, you probably want UPDATE_BATTLEFIELD_STATUS after all, but you need to do an additional check to see whether the event is relevant:
Code:
f:RegisterEvent("UPDATE_BATTLEFIELD_STATUS")
f:SetScript("OnEvent", function(self, event, index)
if GetBattlefieldStatus(index) == "confirm" then
t = PROPOSAL_DURATION
self:Show()
else
self:Hide()
end
end)