It is always dangerous to generalize. Obviously there are cases where JTAG access is next to impossible. But there are just as many (or more) cases where it is possible. Here are some ideas that can make it possible:

1) Simply add an externally accessible connector the the product to host the JTAG function.

2) Make a connector like a 10 way 1 mm flex strip connector accessable in a battery compartment or behind a service plate.

3) Add the necessary signals to spare pins on an existing connector. In this case the system 3V and/or GND connections may already be present on the connector.

4) Multiplex the JTAG control signals with other signals on an existing interface. This can take surprisingly little circuitry and a simple PLD will suffice in many cases. You simply add a single pin to the external connector that is normally pulled high and a jumper in the reprogramming cable pulls it to ground to enable the small PLD to divert the other interface signals to the JTAG function.

Note: In the case of the Cygnal JTAG function the minimum number of signals required are:
+3V - supply power to RS232 -> JTAG pod
GND - Common connection.
TCK - JTAG clock to target
TDI - JTAG data in to target
TDO - JTAG data out from target
TMS - JTAG mode select to target

..so it can be done with just 6 connections.

Michael Karas