why cant we have a standard JTAG cable?
as far as the cable, why not, that is insignificant. What typically "blow" the standard cable is the connector, there is no standard for JTAG connector e.g. my Xilinx are programmed by Xilinx software to a 7 pin one row, my SILabs are programmed by SILabs software to a 10 pin 2 row. In both cases this was chosen to allow all to use the stuff "out of the box". The Xilinx is not on the same board as the SILabs, so, in my case, who cares.

Somewhere in the SIlabs litterature it states that you can have other devices in the JTAG chain, if you find that document maybe it can give you some hints. I am, however, sure that the "file" transferred to these "other devices" has to be massaged by software provided by the vendor of the "other devices".

In other words, you can chain the hardware, but for the specific device you need the specific "file massager". SILabs goes a step further, their EC adapter include processing, thus no other interface can program/debug SILabs chips.

Erik