Recently I modded a F91W-3 with a negative display. There are many resources on how to do this so I won't cover that here. However I want to call out a few points that might help others considering this mod.
1. The mod itself is very easy if you take your time. The most difficult part is removing the glue left behind from the original polarizing film. It took about 10 minutes of constant rubbing with a paper towel soaked with 99% isopropyl alcohol. At first it seems like the display is ruined but eventually it all comes off.
2. Re-using the original polarizing film is a bad idea for two reasons. First, when rotated, the original film is the wrong size. Second, after removing the original film, the adhesive backing is very rough and would look terrible if you simply stuck it back on the glass. To make it look decent you would have to clean off the glue on the original film and then apply fresh glue (not sure what kind). In any case it's much easier and quicker to use a new polarizing film.
3. The polarizing film I used was purchased on ebay and sold as an iphone replacement part. I'm not sure if this is the best type of polarizer to use, but at 2 USD it was the cheapest I could find. The main thing that surprised me about the film was that the polarization axis was not aligned with either edge of the film. In other words, cutting out a square from the top-left corner of the polarizer resulted in poor contrast. A 45 degree cut-out was also sub-optimal. The most optimal contrast angle was somewhere between 50 and 60 degrees, which was annoyingly in-exact and hard to measure with precision. There might be better polarizing films out there that are easier to work with. I'd be curious to learn if this is so.
4. As for the finished product, it looks very cool in optimal lighting. However in average or low light settings, the readability is significantly diminished. For example:
5. The light on the F91W is weak to begin with, but it's even weaker with the negative mod. In absolute pitch dark, the light does make the digits readable, but overall contrast is still diminished.
6. If you wish to undo the mod, the adhesive left behind from the iphone polarizer is much easier to remove compared to the factory adhesive.
That's all! Hope that helps someone.
(Apologies for lack of actual images/links. It won't let me because this is my first post.)