Do you know the model number of the mini PTZ you are thinking about using? I've used a few of the DS-2DE4A220IW-DE which is called a 'Network IR mini PTZ' although it is still considerably larger than a standard Anti-Vandal dome camera. This camera is an external PTZ but I find it fits well in warehouse type environments.
This particular model is not classed as a smart-tracking PTZ (the smart tracking models start DS-2DF***) but it does have a smart tracking mode which can be enabled within the camera settings and works well. It has 50m IR, 20x zoom and can be powered from hi-POE or 12VDC.
Like I say though, the size of this DS-2DE4A220IW-DE would not make it very discreet in a restaraunt environment.
The internal DS-2DE4220W-AE3 (very similar and possibly confusing model number alongside the model above) would probably suit you best if you have a space above the ceiling for the 'in-ceiling' mount and would be more discreet, I can't really comment on installation of one of these though I'm afraid.