I suggest a new tag for the display block, maybe named "<projection>". This takes the video overlay and uses GL projection to move it from the display block, in the direction the display block is facing, and paints a mirrored copy of the display overlay on whatever surface is across an open space from the display block.
How the display block looks normally:
Display block "projected" against a flat white armor wall:
Display block "projected" against a wedge with the narrow edge oriented down:
Display block "projected" against a wedge with the narrow edge oriented stage front:
This kind of "projecting" solution would solve which way the text looks, regardless of the orientation of the wedge. Plus, it would allow you to put the display output on any surface you choose. It will not require any new blocks, and will keep the original display block as the source of this solution, so any enhancements and changes to the display block will propagate to this solution. Plus, projecting the texture like this should work on absolutely any shape the builder desires.
Unfortunately, if the builder makes a single surface out of a hepta and tetra together, only the closest block (typically the tetra) will get the display projection, which would be kinda weird, but it would be a necessary bug of this solution. (Maybe a future bug fix could search for this particular situation and double-project the display texture so that it shows up on both blocks, just like a real projector would.)