Just joined this thread.
I have the same problem with an LG 47LE5400 (2010) TV, yet a newer, cheaper 42LV4400 DOES have variable analog output.
Exact same issue: I want to control external speaker volume using the Tivo remote.
I have not tried the headphone output.
Does anyone know if I can set the Tivo remote to control volume of an AV receiver yet have the TV PWR button control only TV on/off?