By comparing the lsmod output from a recent Gentoo LiveUSB (which handled the touchpad fine) and my sorry mess of a .config, I found that this option needed to be set:
CONFIG_I2C_HID_ACPI=y
( Linux Kernel Configuration → Device Drivers → HID support → I2C HID support → HID over I2C transport layer ACPI driver)
It looks like doing so also enables this option:
CONFIG_I2C_HID_CORE=y
(on top of the settings in the Gentoo wiki for Framework Laptops)