Actually, a reed switch isn't a bad idea. They might be a bit large for a laptop drive (Even the miniature ones) but it might just work.
Perhaps you could use a phototransistor or photodiode (Should be much smaller, especially if you could find a SMD version) and just arrange a LED to shine at it from the inside of the case?
This means that you wouldn't have any problems with a stray magnetic field (Near drives/data cables, etc) or vibrations ejecting the tray, but it might take some tinkering, especially if you plan to have a see-through case, and it would need some testing to ensure that a photodiode/phototransistor would be suitable (Not sure how the button is monitored electronically).
Ah, something like this, perhaps (Heh, this one's even IR filtered):
http://www.rapidonline.com/Electronic-Components/Optoelectronics/Photodetectors/Silicon-pin-photodiode/29403/kw/photodiodeWhichever method you use, you’ll need to make some pretty big modifications to the drive enclosure to fit this extra hardware in and to enable the sensor to be actuated, unfortunately.
Just a thought,
Hodgkinson.