Again, for class A nodes without an uplink you practically cannot, because that is not how LoRaWan works.
Unless you have class B or C node behavior, no, they are not listening. They only listen in the receive window right after transmission. You can send a downlink request for a node to LoRaServer, but it won’t be sent over the air until the target node transmits, indicating the tiny slice of following time when it will be receiving.
Otherwise, if you still have the device address and session keys you are free (at least bypassing LoRaServer) to encode and transmit a packet addressed to that device - but it will not hear you, unless it is receiving at the time, frequency, and spreading factor you transmit. And a class A node would not be, unless you are transmitting in direct reply to an uplink.
If you have non-standard node behavior, you can do whatever you want - and the topology of having an MQTT broker in the middle which exposes everything at network as well as application level makes it easy to experiment. But then it is up to you to make the pieces of your scheme work together.