For applying a force, you might adapt a D'Arsonval current meter. You should
get a linear relationship between force and current. There was a company making
microbalances... Mettler... that I believe used such a device at the heart of
its balance. If you observe the end of the beam with a microscope (on a video
monitor) you should be able to directly observe the deflection. In the event an
observation cannot be made in this orientation, you might consider a fiber
optic sensor which both provides light and measures the reflected return light.
An evaporated gold plating might be necessary to create a reflecting surface.
Of course, interferometry would be best for small displacements. I'm sure
someone in this group has developed a small interferometer.

Bill Lapson