Vibration sensors? EMG?

Good vibrations?

http://yanxinyan.com/tactile_teacher.html

Hsiao, C., Li, R., Yan, X., & Do, E. Y. (2015). Tactile Teacher: Sensing finger Tapping in Piano Playing. In Proceedings of the 9th Conference on Tangible, Embedded and Embodied Interaction (pp. 257–260). Stanford, California, USA.

https://www.academia.edu/10232691/Tactile_Teacher_Sensing_Finger_Tapping_in_Piano_Playing

In the paper, they report 100% accuracy for the thumb in melodic playing with the vibration sensors attached at the wrist. This means no glove would be necessary. This may be the way to go to augment the vision system, for melodic playing at least.

EMG? Meh.
http://research.microsoft.com/en-us/um/people/dan/muci/Saponas_EMG_CHI_2008.pdf
http://www.cs.washington.edu/research/projects/aiweb/media/papers/Saponas_EMG_UIST_2009.pdf