You could print something on paper and take a macro shot of it onto a 35mm film. You could lay this film on a light box and take a macro shot of that and end up with a positive image. I've done the last half myself and actually ended up with a usable and high-resolution image. I used two rolls of ordinary Tmax100 for this purpose. I'm pretty sure you could photograph a normally printed 8 1/2 x 11 paper and end up with enough detail to read it later.

Your problem will be that it is not a reflective image, so it'll be really hard to read even with a microscope. The film may not survive being in the locket for long time either.

I'd keep it simple. A photograph, a few chosen words that mean something to both of you, etc.