This will probably work:
Code:
local function OnDoubleClick(self, button)
LFGListSearchPanel_SignUp(self:GetParent():GetParent():GetParent())
end
for _, button in pairs(LFGListFrame.SearchPanel.ScrollFrame.buttons) do
button:SetScript("OnDoubleClick", OnDoubleClick)
end
You can use
http://addon.bool.no/ to turn it into an addon.