RTGs lose power rapidly as the isotopes decay, and any sort of communication over those distances requires massive power. The Voyagers are essentially dead due to this issue, and they haven’t been out there nearly that long.
It would likely need a standalone fission reactor that only ‘goes hot’ when it arrives.
I’m not sure that we have the engineering ability to actually do that with any real chance of success after a 100 year deep space flight, or the willingness to wait that long to find out.
You still need matter to carry the heat, and matter is heavy (by definition!) Radiating the heat isn't going to be enough if you want to send anything other than a blob of plasma.
Which is an interesting thought when considered as a weapon. Fire a self-immolating fission reactor at your target...
