/etc/modules seems to be loaded by /etc/init/module-init-tools.conf. The first argument is the module name, other arguments are the parameters. Adding the following to /etc/modules seems fine:

thinkpad_acpi fan_control=1

To load this module and set these parameters in the very early stage of boot, add the previous line to /etc/initramfs-tools/modules file. After a change in that file, you need to regenerate the ramdisk:

sudo update-initramfs -u

As a possible alternative, you can try to add the options to the kernel line (I haven't tested it myself, but it seems to work for settings like i915.modeset=1. Edit /etc/default/grub and find the line with GRUB_CMDLINE_LINUX_DEFAULT="quiet splash". Replace it by something like:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash thinkpad_acpi.fan_control=1"

To get a list of options for a kernel module:

modinfo -p thinkpad_acpi

This did not work for i915, for that I had to run:

modinfo i915 | grep ^parm

To get the current value of a module parameter, run:

sudo cat /sys/module/MODULE/parameters/PARAM

For the fan_control parameter of the thinkpad_acpi module, you have to run:

sudo cat /sys/module/thinkpad_acpi/parameters/fan_control

If this function returns an error, check if the module was loaded and whether the option exist or not.