After being annoyed by the lack of (mostly lateral) view movements in first person for some time, I figured out a way to alter it externally.
This might be interesting for everyone without VR or headtracking equipment.DemoInstructions
Get OpenTrack 2.3.9 (newer versions are bugged) (https://sourceforge.net/projects/opentrack.mirror/files/opentrack-2.3.9/opentrack-2.3.9-win32-setup.exe/download), select Joystick as input, freetrack as output and EWMA as filter. In GPB, activate Free Look and select TrackIR. The resting position of the view can be adjusted by pressing Shift+C on track and using the arrow keys. These positions are independent between the two FPVs and specific to each bike. I suggest using the first view (linked to the rider).
You setup the axes and mapping by clicking on the hammer icon next to "Joystick input". The moving squid will show you if you mapped it correctly after clicking "Start". Auto rider lean players should use their steering axis as input and map it to the lateral L/R axis (X), while manual rider lean players can use their rider-thumbstick (probably the right one) for both the lateral (X) and longitudinal F/B axis (Z). Axes can be inverted in the "Output" tab under Options.
You can also manually override GPB's pitch, lean heading (Yaw) or tilt (Roll) through OpenTrack, allowing for full control independently of the bike. This might require some additional practice, though.
I suggest setting the EWMA filter to 20-40% (both Min. & Max.) with "Curve" at 10% (it doesn't really make a difference). The movement range and linearity can be adjusted through the mapping curves but be careful to only edit the curve of the largest "Max translation". I recommend keeping it completely linear, possibly with a slight deadzone at the start.
Calibrate the joystick by enabling "Center at startup" in the options, then clicking Start and checking the outer ends of the mapping curves while moving the stick. If they don't line up properly, stop tracking, slightly move the resting/0 position of the stick and restart tracking. Once you have it properly calibrated, make sure to deactivate "Center at startup" in the options so that these calibration settings stay permanently.
Bind both "Start tracking" and "Restart tracking" to the same button in the OpenTrack options for when GPB randomly resets the FreeLook mode.
I attached two INI-files for a standard Xbox360 gamepad. Put them into the opentrack folder under "My Documents". You can use the mapping curves as a starting point for your setup. Don't forget to calibrate your joystick when loading these profiles!Explanation
You are basically sending fake headtracking data to GPB which allows you to freely move and rotate the first person view beyond the default limits.
GPB handles this little hack very nicely, in that all view settings that are not being manually overridden by the player through OpenTrack are kept to the user defined values. So when only mapping the L/R (X) axis (which is all that is needed really), the default F/B view movements and all of your settings (pitch, tilt, lean heading) remain. This means that you don't have to completely relearn first person riding, you just have to get used to the bike no longer being visible in corners like in reality.