Self-answering - yes, it can be done rather trivially.
You'll need the following modules installed:
kmod-hwmon-core
kmod-hwmon-gpiofan
kmod-i2c-core
and (assuming a standard official Pi 4 fan on GPIO 14) an entry in /boot/config.txt that looks thusly:
dtoverlay=gpio-fan,gpiopin=14,temp=80000
(these are the Raspberry Pi Foundation defaults, obviously adjust gpiopin if you've moved yours and temp to taste)
Reboot and enjoy the thermally-dynamic active cooling!