I'm currently on Havoc 3.5 GSI, with Trinity kernel and MR3 vendor. Can confirm that hid_multitouch is built into the kernel according to the kernel config. However, I couldn't find anything about how to configure the external touch panel and gravity sensor, despite both of them have been properly detected by the system:
1. A CH340 USB-Serial port (1A86:7523) which I believe is the gravity sensor. This device can be picked up by apps like Serial USB Terminal.
2. A Silicon Works SW4101C (29BD:4101) which is most likely the touch panel.
Some information online suggests me to create an IDC for the touch panel's device ID (at present it's identical to that of qwerty2.idc, except I now specified the device is an external device), and some even suggested to patch the kernel's hid-multitouch.c to add the device's ID and information into it, but hid-multitouch.c suggested otherwise, that a default method would be used for unspecified HID devices (that only devices known to have some quirks should be included along with the code for the quirks). I checked the stock ROM's /system/usr/idc and found nothing about external touch panels, either. (If anyone has experiences with such on stock ROM it'd be appreciated)
The gravity sensor is not really working at all (maybe this requires a specialized USB serial app). For the touchscreen part, I could at best get working touches from upper-left part of the screen (about 2/3 of total screen, I think), and only when the phone is in landscape mode (in portrait mode the touchscreen does not function at all). However, it seems sliding works on the entire screen as long as the touch is started from the area that has working touches. Touch is still unstable as some incorrect touches may occasionally occur (probably cable/power related). Also, my phone would generate a tremendous amount of heat when the monitor is attached, and doesn't seem to be able to boot properly if it's been overheated (like about 60-70 celsius).
Not sure if external touch panels are still new, or that such is not really practical for Android smartphones to use (compared to desktops and laptops), the information I could find about this is very limited (that it might require some vendor-specific system modes to function properly, which Huawei and Samsung phones seem to provide). I intend to use this panel as a substitute for my Pixel C which is now showing its age, as it could not handle certain recently released games very well anymore.