So, I played around with the driver's source code for a few hours, and
managed to get the touchpad to work. It turns out that my previous guess
was wrong: this device doesn't use the v3 protocol, but v2 with some
small modifications with respect to what is supported by the driver.
Attached is a very crude patch that I made against Seth Forshee's dkms
driver, downloaded from http://people.canonical.com/~sforshee/lp681904
/psmouse-elantech-dkms/v0.2/. The patch is not intended to be integrated
as is, because it will surely break other devices, but it should be
enough to show what I did.

I found two main differences with respect to the currently supported v2
protocol. First, the resolution values currently hardcoded in the driver
don't work for this device, which seems to have a lower resolution.
Adjusting these values after some trial and error fixed most of the
issues. Also, the device sends packets every now and then that have both
coordinates set to 0xff. These packets have to be ignored (or treated
properly, but I cannot guess what their purpose is) in order for the
touchpad to work.

With this patch applied, my touchpad is working flawlessly. In
particular, two-finger scrolling works correctly, which used to be
completely unusable before. There's of course some device detection code
that still has to be written, but that seems to lie beyond my level of
expertise, as I have no experience with kernel programming. Still,
please let me know if can help with testing.

** Patch added: "Proof-of-concept patch"
   
https://bugs.launchpad.net/ubuntu/+bug/805818/+attachment/2387625/+files/elantech.patch

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https://bugs.launchpad.net/bugs/805818

Title:
  Mouse Erratic Movement not able to control.

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