There is an existing feature request in Fider that I would encourage you to upvote:
Access to the slices created by using the 3d slicing feature · LightBurn.
To select a near-equivalent of the last pass, use the Trace Image tool and move the ‘Threshold’ slider all the way to the left to zero.
Move the slider to the right to include more in your background.
If you had a 3-pass 3D Slice job, then you would move the Threshold slider to 1/3 of 255 to get the last pass.
Use the resulting trace as a fill and re-process the last pass however you need.