Gonna grab a comment from Hotrian on reddit as a possible solution to paste here. Hopefully it helps! It’s 9 years old, though hopefully it is either still viable or some else will provide a more updated method.

case you don’t read the other comment I made,

The correct solution is probably to just use OpenVR-InputEmulator to just rotate the HMD orientation. Then OP can lay down but face “forwards”, and the tracking system is still properly calibrated. I use this all the time when I’m developing and need to move my controller somewhere I physically cannot. This doesn’t solve the inner ear issue but with a bit of tweaking OP should be able to reposition and reorient his devices so they treat “left” as “down”, or whichever direction he needs them to. The Device Offsets page allows you to redefine device orientation.

Trying to trick the lighthouses (or rather the LH System) on the physical level is a bad idea as there is some pretty complex math going on. Instead I would recommend just redefining how SteamVR interprets the device orientations.

I haven’t tried redefining the down direction, but I offset my devices and rotate around all the time, and I think you should definitely be able to do it with OpenVR-InputEmulator.