Jay Shelton sent me a link to a YouTube video showing how to liberate a baseball card from a PSA grading slab lat night and I decided to try out their technique on my 1973 Shigeo Nagashima card. It was pretty straightforward - all I really needed to do was cut the corner off the top section then use a flat head screwdriver to pop the front and back of the case apart. Once I found something that could cut through the top part it took about 30 seconds (like the video says) to do it.
Here's some photos of the liberation process:
Thanks for the link Jay!