Okay, I understand the problem, but there might be a solution :
By asking users (via a dialog box) to select the painting mode for their model when creating the project, so:
Choice of painting mode:
1/ PixZels method (1 color per face, i.e., 6 faces) => .VOX Export or Slice Export button disabled
2/ Voxels method (1 color per cube) => .VOX Export or Slice Export button enabled 
It’s up to you to decide which is less complex to implement.
Sincerely,