I'm not sure anyone else is actively working on this, but this is what I have found so far.
I've been doing all my development on real hardware. By chance, all computers that I have access to happen to have the NEC/Renesas USB chipset, uPD720200 series. None of the computers ever had any success what so ever with the xhci driver.
I added a bunch of debug code, and found that the Noop() works consistently, so Command Transfers are working fine.
However, the driver would always fail trying to get the device descriptors.
I found a bug, and now the driver is able to complete getting the device descriptors for connected devices.
There's still a problem with the buffer and TRB completion handling that causes transfers to stop at some point after.
I had no previous USB experience, but since you posted this a few months ago, I've been digging in and learning. I'm going to continue working on it and hopefully get it working.