I am having a similar problem with my Direct TV HD-DVR and SVS sub. It is definitely coming in through the sheild of the two satillite feeds into the DVR (when I remove them the hum is gone). This confused me at first since the only audio path to my receiver (Denon 3805) was through optical connetions for both the HD-DVR and OPPO DVD player. The video paths to a Sharp LCD and Panasonic Front Projector are HDMI where all the switching and distribution is done external to my receiver. The only RCA connection to the receiver is a composite video out to support on screen displays for receiver set-up...... this was the path where I was getting the noise thru.
The HDMI cable from my HD-DVR was feeding Sharp LCD which was also connected to the monitor out from the receiver. I rationlized the noise was coming to the LCD from the DVR via the HDMI cable and returning to the receiver via the monitor out cable. I removed the monitor cable (with the sattilite connection to the DVR still in place) and the hum disappeared. Since I don't use the OSD very often, I can leave it disconnected for the most part which solves the problem (in a band-aid way) for me.
Like you I searched for an in-line isolation transformer for the satillite side and found none. I'm thinking now that the only way I may be able to solve the problem will be with a line level transformer between my receiver and sub. Let me know if you find a solution that works for you.