This wouldn't be the first ThinkPad with a transparent display. The ThinkPad 755CV/CDV[0] let you lay the screen on a light projector to become an LCD projector.
Wow the overhead projector. Such a blast from the past. This is when things got even more boring than the death by powerpoint of today :) usually black and white and a nerdy professor speaking in monotone for a whole hour or more.
[0] https://www.thinkwiki.org/wiki/Category:755CV