Well this looks doable with Lua, as an http action probing an api that would let it now if the destination service is available and where. One point about routi

Well this looks doable with Lua, as an http action probing an api that would let it now if the destination service is available and where. One point about routing, do you mean that server for handling /foo/bar may not exist yet in haproxy's configuration? If so, we could enforce the destination IP at runtime, based on the response provided at the step above.