PS4 Controllers can be connected via BT to phones and via cable to PC with almost all functionality. There's an extension port on the controller but I haven't seen anything that plugs into it. Also don't know how easy it is to connect your own controller to the PS4 but there is an "ext" port on the controller that is reportedly a USB2 port but a proprietary shape.

This is true, unfortunately. A common way around that is that you must connect both the controller you want to use and an original PS4 controller to the adapter. The adapter will forward all DRM/handshake messages to the original controller, but forward the input from the desired one.

Unless Sony is now signing every message I see no reason why this approach couldn't be used on the PS5/DualSense. That said, my cursory search didn't reveal a working PoC.

You probably aren't remembering correctly: the protocol for DualShock 4 (PS4 controller) was reversed - there's a working userspace driver[1] that I have used for years to play games on Linux using a DS4 controller, connected via Bluetooth. More radically, Sony recently published an official Linux driver[2] for DualSense 5 - none of these require a "DRM chip" or additional hardware to use.

1. https://github.com/chrippa/ds4drv

2. https://www.phoronix.com/scan.php?page=news_item&px=Sony-HID...

More interesting is that whilst the original PS3 controllers had a full Bluetooth stack (meaning you can connect them to an Android device or Linux and they work perfectly fine), the bootleg clones only implemented the spec enough to get them working with the PS3. I didn't even know this until I tried to connect my old PS3 controller to my Pi running retro emulators, and all the controller would do is input to bottom right on the right analog stick. Same on Android.