It's mentioned in Wikipedia (Japanese) together other HD interfaces but that's all --> http://ja.wikipedia.org/wiki/MSX I tried to find more info but unsuccessful. Help should come from Japanese users then...
Previously I heard that:
Some people ordered the IDE interface, and sent a money. However, the hardware was not delivered.
Some people ordered the IDE interface, and received the hardware and FD. However, it could not be used because FD was empty.
Some other people could use the IDE interface. However, the driver may have some bug.