1. Download USBTabletINIT.sit from https://github.com/kanjitalk755/macos9-usb-tablet/releases.
2. Expand it and move to System Folder of guest MacOS9.
3. Append "-device usb-tablet" instead of "-device usb-mouse" to argument of QEMU.

The project file was made by CodeWarrior Pro 6.
To build:

1. Expand icon.rsrc.sit
2. Change file creator of USBTabletRSRC.prj and USBTabletINIT.prj to 'CWIE'.
3. Open USBTabletRSRC.prj and build.
4. Open USBTabletINIT.prj and build.

Do not add via=pmu to the QEMU argument -M as it will not work properly.

Here's another technique with a similar purpose:

https://github.com/elliotnunn/macos9-usb-tablet