so, I put this together - The host (testing with my PX) detects it as a mouse, but I don't get any mouse movement...

#!/bin/bash

set -e
set -x
set -u
modprobe libcomposite

cd /sys/kernel/config/usb_gadget/
mkdir -p g1
cd g1

echo 0x1d6b > idVendor # Linux Foundation
echo 0x0104 > idProduct # Multifunction Composite Gadget
echo 0x0100 > bcdDevice # v1.0.0
echo 0x0200 > bcdUSB # USB2

STRINGS_DIR="strings/0x409"
mkdir -p "$STRINGS_DIR"
echo "6b65796d696d6570690" > "${STRINGS_DIR}/serialnumber"
echo "tinypilot" > "${STRINGS_DIR}/manufacturer"
echo "Mouse" > "${STRINGS_DIR}/product"

Mouse

MOUSE_FUNCTIONS_DIR="functions/hid.mouse"
mkdir -p "$MOUSE_FUNCTIONS_DIR"
echo 0 > "${MOUSE_FUNCTIONS_DIR}/protocol"
echo 0 > "${MOUSE_FUNCTIONS_DIR}/subclass"
echo 5 > "${MOUSE_FUNCTIONS_DIR}/report_length"

Write the report descriptor

D=$(mktemp)
{
echo -ne \x05\x01 # USAGE_PAGE (Generic Desktop)
echo -ne \x09\x02 # USAGE (Mouse)
echo -ne \xA1\x01 # COLLECTION (Application)
# 8-buttons
echo -ne \x05\x09 # USAGE_PAGE (Button)
echo -ne \x19\x01 # USAGE_MINIMUM (Button 1)
echo -ne \x29\x08 # USAGE_MAXIMUM (Button 8)
echo -ne \x15\x00 # LOGICAL_MINIMUM (0)
echo -ne \x25\x01 # LOGICAL_MAXIMUM (1)
echo -ne \x95\x08 # REPORT_COUNT (8)
echo -ne \x75\x01 # REPORT_SIZE (1)
echo -ne \x81\x02 # INPUT (Data,Var,Abs)
# x,y absolute coordinates
echo -ne \x05\x01 # USAGE_PAGE (Generic Desktop)
echo -ne \x09\x30 # USAGE (X)
echo -ne \x09\x31 # USAGE (Y)
echo -ne \x16\x00\x00 # LOGICAL_MINIMUM (0)
echo -ne \x26\xFF\x7F # LOGICAL_MAXIMUM (32767)
echo -ne \x75\x10 # REPORT_SIZE (16)
echo -ne \x95\x02 # REPORT_COUNT (2)
echo -ne \x81\x02 # INPUT (Data,Var,Abs)
# vertical wheel
echo -ne \x09\x38 # USAGE (wheel)
echo -ne \x15\x81 # LOGICAL_MINIMUM (-127)
echo -ne \x25\x7F # LOGICAL_MAXIMUM (127)
echo -ne \x75\x08 # REPORT_SIZE (8)
echo -ne \x95\x01 # REPORT_COUNT (1)
echo -ne \x81\x06 # INPUT (Data,Var,Rel)
# horizontal wheel
echo -ne \x05\x0C # USAGE_PAGE (Consumer Devices)
echo -ne \x0A\x38\x02 # USAGE (AC Pan)
echo -ne \x15\x81 # LOGICAL_MINIMUM (-127)
echo -ne \x25\x7F # LOGICAL_MAXIMUM (127)
echo -ne \x75\x08 # REPORT_SIZE (8)
echo -ne \x95\x01 # REPORT_COUNT (1)
echo -ne \x81\x06 # INPUT (Data,Var,Rel)
echo -ne \xC0 # END_COLLECTION
} >> "$D"
cp "$D" "${MOUSE_FUNCTIONS_DIR}/report_desc"

CONFIG_INDEX=1
CONFIGS_DIR="configs/c.${CONFIG_INDEX}"
mkdir -p "$CONFIGS_DIR"
echo 250 > "${CONFIGS_DIR}/MaxPower"

CONFIGS_STRINGS_DIR="${CONFIGS_DIR}/strings/0x409"
mkdir -p "$CONFIGS_STRINGS_DIR"
echo "Config ${CONFIG_INDEX}: ECM network" > "${CONFIGS_STRINGS_DIR}/configuration"

ln -s "$MOUSE_FUNCTIONS_DIR" "${CONFIGS_DIR}/"
ls /sys/class/udc > UDC

chmod 777 /dev/hidg0
#chmod 777 /dev/hidg1