David, this is awesome! Although I like my IR sensor, it has its limitations, and the contact solution like this would be great. However, I have a question. How this will accommodate existing mounting for arms? It does not look like the PCB has a lot of space to mount something like Rostock ball-cup attachments or u-joints, unless they are mounted on posts. The problem with switching arms is that it would also require switching (or adjusting) attachements on the carriage side, or possibly changing carriages. While this may be ideal for some people (as you have noted you have eliminated the tilt buy changing the arms/carriages), it does add to the total cost of the solution, as well as the time involved.
As for the weight of the PCB vs Al effector, I wouldn't think that a PCB is necessary lighter than Al plate such as http://713maker.com/effector.html or a similar from http://tricklaser.com. The 713 effector (without a mount) weighs in at about 10g and Rostock ball-cup plastic effector (without a mount) about 13g. Sure, there is an added weight of the mount, and PCB-only plate might be lighter, but I do not think it is really that important here, as the weight of E3D extruder is about 50-80g or even more for some models.
Regardless of the weights, I would propose separating the effector from the mount part. This way, people will be still able to use their current effectors/arms/carriages/rails without modification. I understand that it will require additional work for you, but it would save a lot of work and money for those who are not ready to buy the whole set. -Thanks.