In our use case, we have a touchscreen that we do not want to affect a master pointer. For that reason, the touchscreen device has been set to act as a floating slave in XInput (not attached to any master pointer). However, we still want to receive multi-touch events from the device in our Qt widgets.
The device discovery code in QXcbConnection::xi2SetupDevices() skips any device that does not have the 'use' field set to XISlavePointer.
We have resolved this problem by modifying the if-statement in QXcbConnection::xi2SetupDevices() to not discard pointers with use XIFloatingSlave on top of the regular slave pointers.
I don't know if such a change would have any adverse effects in other situations, but I don't see why you would want to ignore events from floating slaves.