Circumvent the Harmony remote device limit with this hack (pictures)

[Optional. Perform this step only if your phantom device needs to be turned on and off along with the activity. For some devices like Roku that stay "on" all the time, this step isn't necessary.] Hit "Activities" on the left menu, select the new activity and hit "Customize this activity." Under "Activity startup commands" choose "Add step," select the piggy-backed device, choose "IR Command" under "Action," and the new command for Power on your phantom device under "Value." Repeat for "Activity shutdown commands."

Sync your changes using the yellow pull-down in the upper right. You're done!