I did say without rebuilding.
By editing, or replacing the wb_presets.json text file.
You just won’t be able to select e.g. exact “Tungsten”, “Daylight”, or “Shade” etc. WB coefficients that match the ones you could set in the camera and OM System think are right for this camera under particular illumination. You will still be able to manually play w/ the WB coefficients/tint/temperature sliders to your heart’s content in darktable (or use the automatic “as shot” WB that the camera detected and reported), but just won’t be able to “preview” what a particular camera preset would have given you with a single click if you had hypothetically set the camera like that during the shoot.
But we digress - this topic is about dt 4.2.1, not about the OM-1.