The BCM CPU used on the Series4 platform has a "factory mode" that is disabled at time of production. Once this is disabled by blowing a fuse on the CPU, it also kills JTAG.
Currently the only known way of re-enabling JTAG is to somehow obtain a "virgin" CPU and doing some advanced BGA rework to replace the CPU on the board. Even then, the TiVo software is programmed to detect the presence of factory mode and it does "things" in response to this.
All that said, if you do somehow get JTAG to do something useful on the Series4 please don't feel shy about posting it. :-)