The PW2 and PW3 appear to have a pair of testpoint pads, conveniently located between the serial port pins and the (missing switch) "USB BOOT" pads. There is a "JTAG" label above one of these two testpoints, and no other nearby testpoints (so they are probably both JTAG pins).
As it turns out, there is a "
reduced pin count JTAG" that only uses two pins: Test Serial Data (TMSC) and Test Clock (TC), plus a ground.
And there is even a tutorial available for "
Inexpensive JTAG on the imx6 solo: U-Boot Launch and Debug", which should be close enough to the PW2/3 (and newer models) to work without too much hassle.
Beware that this has yet to be tested on a Kindle, and if it releases the "
magic smoke", that would be bad...