I have the trackpad working as a mouse! I will try to make a video of it, but it is difficult to show both the trackpad being used and the pointer moving on the screen. But, it indeed works. I'm amazed on how sensitive and precise it is. I'm actually able to move across the whole screen without much effort and at the same time, make small and precise movements. I'm not sure if Linux is adding some sort of acceleration to the pointer, but it works perfectly.
At the moment I'm polling the sensor via I2C since my interrupt line is not being converted to 3.3v (I'm reading the 1.8v line directly) and it does not work reliably as one would expect. Fixing this will eventually enable low-power reading of the sensor.
With this result I'm actually deciding on going ahead and design a PCB with everything needed for a wired version: USB connector, the two power supplies, logic-level converter and MCU (not sure if STM32F103, I might go with some STM32L MCU to eventually have a low-power design ready for a wireless version). I will also need some way of reading button presses. I will probably better to have capacitive touch buttons than mechanical ones, since I imagine it will be more comfortable. Also, I need to design this so that the device is not too big and I can design a part to be 3D printed to mount the whole thing on a finger.
Anyway, there's a slight issue with this: I need to find an appropriate connector for the sensor. I found some shady vendors on strange sites, selling blackberry parts. I would like to find something that I can buy from a reputable source such as digikey or similar ones. I don't really know if this kind of connector is supposed to be more or less standard or if it is custom-made for blackberry (seems unlikely). If you could help identifying I would be really thankful!!
Here's a picture of the BlackBerry 9800 sensor (the one I'm using) on the left, and a 9900 sensor on the right. It would be cool to find both, but I need at least for the 9800 one: