aha !

It seems the ST DCU3 unit is lockable on most of the 5514/16/17 devices (and probably others as well). From what I've been told the unit can either be locked permanently or put into a mode where it requires an unlock key (which seems to be customer definable).

When locked, the DCU unit will ignore virtually everything except the unlock command. The TDO pin will tend to stay in its 3-state hi impedance state whenever the DCU should be outputting data. This behaviour is pretty much what i've been see'ing on my test board

DCU lock doesn't interfere with the jtag ID command.

Looks like the only way to jtag these boards might be to replace the processor with an unlocked chip. Probably not really very practical for the majority of people.