[nitrogen]

Unless there is some way of encrypting data all the way to the display controller I don't know about.

That's kind of what HDCP is, so it could be done on an embedded display too.

[SomeOldThrow]

IIUC you'd have to encrypt the text rendering, not the rendered text, for the message to be e2e encrypted and inaccessible through memory.

[nitrogen]

In theory you could run the decryption and rendering in the GPU (as is done for video), which would re-encrypt for display.