Remember also that a standard webcam can only show you bright objects. In order to track like you suggest, the camera would need a high frame rate and sensitivity, and these usually run counter to one another until you get to very expensive hardware.
I have an SBIG STV (about $1,500) that can provide several updates per second for objects down to about Mag 6 or 7. However, that still is not rapid enough to accumulate enough light to see these objects well, much less track them, until it's accumulated about a dozen such images. By that time, with any reasonable focal length (and the corresponding smaller field of view) the target will have left the field of view.
I've also used a Stellacam EX in situations where I needed to get an image in minimal time. Like the STV, it can provide an image of objects like M4 or M13 -- even M57 -- within just a few seconds. However, without a tracking mount the image is either blurred due to motion or gone from the FoV.
If you have the electronics and software skills, you could probably build a camera based on a chip like the Sony HAD CCD and write software to do what you want. Even so, unless you had very large aperture (minimum of about 14") you would not be able to gather enough light to make this practical.
Hmmm ... I wonder what the STV could do if I used it with an 18" or larger Obsession ... 