probably u can just take a small, flat head screwdriver and manage to pry open the plastic casing. Not sure what that would do to the mouse itself, but its the best i could come up with off the top of my head.
I know this may be an exhausted option, but have u tried compressed air?
Yea compressed air may not be powerful enough to get into the cracks to dislodge the dust. I would suggest a commecial ari compressor, but that would cost more than a new mouse itself!
If compressed air doesnt work and taking it apart is too risky for you, i suggest a new mouse is in order.