Hello Boon Jin, Johnny Lee,
Maybe you might be interested in this idea: with 2 wiimotes, it should be possible to track an IR pen in X and Y with 2 wiimotes place under (or over) and to the side of the projected display. A bit like this:

This way, it would alleviate problems with the wiimote being too far away from the display (or the IR LED being too weak), the LED would possibly be more visible to the IR sensor from the side than from the back, and the presenter would never have to worry about hiding the IR pen from the wiimote. On the other hand, it would require 2 wiimotes, and the tracking would worsen as the pen gets further from the X and Y sensors.
I reckon it would be a nice additional tracking option to have though.