Thank you for your reply - and for correctly recognizing that Arch Linux doesn’t seem to have the steam-devices package available yet. I added the rule you provided as /usr/lib/udev/rules.d/71-dualshock.rules - and it worked! I can now get both gyro and accel data.
The only remaining oddity is that now, when I call SDL_JoystickOpen(0), the following line appears:
ioctl (GFEATURE): Input/output error
But it seems to be working correctly despite the error, as far as I can tell.