The biggest problem with this approach is that, above self resonance (where most of the energy is pumped into a speaker) it is an force or velocity device, not a position device. That is, the force applied to the mass of the speaker (and the considerable mass of the air in front of it) is proportional to the current delivered to the speaker. So there is a considerable phase shift between the speaker current and the position of the cone, as well as an integrator type frequency response (reduced excursion at higher frequencies for similar current). So any servo type of drive has to take that into consideration. It is the pressure (fluid force) that is ideally close to being in lockstep with the driving waveform, not the position.
Of course, a close microphone could provide nearly real time pressure feedback.