I have measured the piezoresistive coefficient of the poly1 layer in
MUMPS process to be about -11.5x10^-11 Pa. Check these papers for
information on piezoresistive coefficient of polysilicon in more general
cases:

V. A. Gridchin, V. M. Lubimsky, and M. P. Sarina, “Piezoresistive
properties of polysilicon films,” Sensors and Actuators A: Physical,
vol. 49, no. 1-2, pp. 67–72, June 1995.

  P. J. French, “Polysilicon: a versatile material for microsystems,”
Sensors and Actuators A: Physical, vol. 99, no. 1-2, pp. 3–12, April 2002.

  L. Latorre, Y. Bertrand, and P. Nouet, “On the use of test structures
for the electro-mechanical characterization of a CMOS compatible MEMS
technology,” in Proceedings of the IEEE International Conference on
Microelectronic Test Structures, vol. 1, March 1998, pp. 177–182.

Hope it helps.

Good luck,
Behraad