A HOT knife and carefull cutting would probably get the case open.  I don't think that anyone has popped one open before this and lived to tell about it!!  HA!HA!

Seriously, a hot Exacto would slice through it easily, we used to cut apart model cars in my early years to do convertibles etc..  Catch is getting it straight around the outside.  You might have someone help you and brace the speedo and rotate it while you cut.  Another small cutter is a coping saw with a fine tooth blade.  You can probaly easily glue it back together after you are done with either hot glue of some of the plastic glues that are on the market.

Good luck and let us know how it goes.

Swiss