Probably nothing. Previous, focused SETI searches have more or less ruled out anything within a couple of dozen light years. I'm pretty sure that after a time delay of centuries, and against the solar and Jupitarian background noise, and feedback from the receivers will be well below the noise floor.

Plus there are hardly thousands of appropriate recievers. We're not talking about shortwave radio here, we're talking about using seriously powerful radio telescopes, which are capable of distinguishing very small changes in power. SETI at home uses the 305m Aricebo telescope which is the largest single-dish radio telescope in the world.

Quote - More seriously, if we do locate a likely signal, what happens when
thousands of eager listeners buy appropriate receivers, tune them
to that freq, and listen at synchronized times?

The senders, if they are as advanced as we like to imagine, should be able to 
pick up the slight change in energy absorption from our direction, or perhaps 
even pick up the IF oscillation from all those superhet receivers on one freq.

We won't need to respond in any meaningful way; they will know we've
picked up the extension, so to speak.

(Actually this technique is delicate but hardly futuristic; both sides used 
it in WWII to determine if their clandestine signals were being intercepted.)