Since nobody could tell me how to do exactly what I wanted, here's the solution I came up with:

Disabling the touchpad/mouse

$ xinput --list

⎡ Virtual core pointer                          id=2    [master pointer  (3)]

⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]

⎜   ↳ FSPPS/2 Sentelic FingerSensingPad         id=12   [slave  pointer  (2)]

⎣ Virtual core keyboard                         id=3    [master keyboard (2)]

    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]

    ↳ Power Button                              id=6    [slave  keyboard (3)]

    ↳ Video Bus                                 id=7    [slave  keyboard (3)]

    ↳ Sleep Button                              id=8    [slave  keyboard (3)]

    ↳ Power Button                              id=9    [slave  keyboard (3)]

    ↳ USB2.0 UVC PC Camera                      id=10   [slave  keyboard (3)]

    ↳ AT Translated Set 2 keyboard              id=11   [slave  keyboard (3)]

    ↳ MSI Laptop hotkeys                        id=13   [slave  keyboard (3)]

As we can see, my touchpad is the one with id=12. To disable it, I need to run:

$ xinput set-int-prop 12 "Device Enabled" 8 0

and to enable it again:

$ xinput set-int-prop 12 "Device Enabled" 8 1

Getting the lid state

First of all, I can get the state of the lid on my system from this file:

$ cat /proc/acpi/button/lid/LID0/state 

state:      open

When I close the lid, the state becomes closed.

Now to get the state as a variable, I can check the exit status of a nifty grep command:

$ grep -q closed /proc/acpi/button/lid/*/state

$ echo $?

1

So to enable or disable the touchpad depending on the lid state, all I need to do is:

$ grep -q closed /proc/acpi/button/lid/*/state

$ xinput set-int-prop 12 "Device Enabled" 8 $?

Hooking the ACPI lid event

To execute the above commands every time the lid closes or opens, I created the file /etc/acpi/local/lid.sh.post with the following content:

export XAUTHORITY=`ls -1 /home/*/.Xauthority | head -n 1`

export DISPLAY=":`ls -1 /tmp/.X11-unix/ | sed -e s/^X//g | head -n 1`"



grep -q closed /proc/acpi/button/lid/*/state

xinput set-int-prop 12 "Device Enabled" 8 $?

XAUHTORITY and DISPLAY need to be set in order to allow root (who runs the acpid process) to access the user's X session.