People have possibly been hacking 'space objects' for well over two decades, but (understandably) not much of that is in cited journals. The most you hear about it is that someone knows someone who knows someone that did something, in very rare cases the someone that did something even has a name.
That's not exactly verification, but there is so much smoke around this subject it would be hard to imagine there are no sparks or even a small fire.
Hacking a space borne piece of hardware that is in active use is probably an excellent way to become a 'person of interest' for a ton of agencies that you want no interaction with whatsoever. If you see an E2 flying in your neighbourhood while you're doing this it might be time to start packing.
The 400W transmitter is doable, the large parabolic antenna would be a bit harder to hide. Though, as you up the power you can use a smaller antenna. The giveaway then becomes fried birds falling out of the sky.
You're going 'up' with a highly directional antenna, and you're going to be transmitting very short bursts. The biggest risk will be the lobes projecting from the antenna some of those will be pointing back at the planet instead of into space. Airtraffic will have the biggest risk of interference (also because the frequency band is close).
So kidding aside you will need to make very certain that (1) you have the right permits and (2) that your gear is not inadvertently sending out a dirty signal. The narrower the better. Better start shopping for a second hand spectrum analyzer ;)
Half of the problem is recieving signals back, as you can't just turn up the power on the satellite. I suppose if you knew exactly what command to send, and there was no form of handshaking this might not be a problem.
We will now have people hacking into Space Objects as the new pinnacle of extreme hacking. Someone is going to do something bad with this new idea.