I've thought a little on this as well, from time to time. Hardware-wise, I don't see how the Quest 1/Lenovo Mirage Solo/Vive Focus were anything much more than Qualcomm's reference design headset (https://developer.qualcomm.com/hardware/snapdragon-835-vr-de...). Different strap designs, minor variations on display modules, removing ancillary stuff like the cellular radio and GPS, but otherwise the same CPU, same GPU, largely the same cameras.
With the Quest 2 running the Snapdragon XR2, a lot of the basic features of 6-DOF VR, with controllers/passthrough/voice control/etc, could be running straight on chip. (https://www.thundercomm.com/app_en/product/1584417287281447)
Qualcomm provides an SDK for VR on their hardware that touts all the things Facebook has always claimed were their own big innovations, like asynchronous time warp and lens corrections (https://developer.qualcomm.com/software/snapdragon-vr-sdk)
I can't find the link right now, but Qualcomm/Thundercomm are also providing a base Android image specifically for use in VR. Which is probably why you can build and install regular ol' Android APKs on all of these headsets and they map the controller to mouse actions.
My point is, I am starting to doubt Facebook has all that much "special sauce". I think it's mostly just marketing. I think Qualcomm has done the hard work here and is happy to let other companies take the credit because Qualcomm is about chip licenses, not consumer electronics. Other than tuning parameters for camera positions, there should be no reason why the OS image for any one of these headsets to run on any other one.
I think it's doable. I think you could probably root the firmware and get an open source OS running on any one of these headsets. And it'd also breath new life into systems whose creators basically got bored with not making a billion dollars overnight.