The power button can be programmed to power on/off a second device. Use the same code that the volume and mute working on the soundbar, and you should hopefully be all set.
googled it, one of many results: (from http://blog.tivo.com/2010/08/guest-p.../#.UtL5zfQ7f0c
"People also seemed to be confused about what the remote controls can do. Start with Messages & Settings -> Settings -> Remote, CableCARD, & Devices -> Remote Control. The TiVo remote can control the power on BOTH your TV and receiver. Program the TV power like normal, then do the procedure again – only this time enter the IR code number for your receiver (maybe it’s receiver first than TV, but the point is it can do both). One press sends both signals. I’ve been using this feature for many years now, it is very convenient. Also, the TiVo Series3, Glo, TiVo HD XL, and Premiere XL remotes are all learning remotes – so even if it doesn’t have the IR code you need, several buttons can be programmed by ‘learning’ from your existing remote"