I don't have a Genie to test anything with, but it's my understanding that the Genie mini boxes do understand shef, but that commands need to be sent to the main Genie unit, which will route them to the mini unit. I believe this is done by utilizing the MAC address of the mini box. Similar to your example, a command to press the channel up button might look like this:
http://192.168.1.104:8080/remote/proces ... ntAddr=mac"
Where 192.168.1.104 is the address of your main Genie STB, and 'mac' at the end is the MAC address of the Genie mini you would like the command sent to.
If that is accurate, direct URL commands could be coded in actions with Python scripts, even if the current Directv plugin can't handle them.
I am currently eligible for a DTV equipment upgrade, and am debating about going to the Genie, or just updating to current HD/DVR receivers. I would hate to lose channel control of my devices...