Hi,
is it possible to define the alpha by inputting a value either through gui or cmd, instead of dragging the alpha bar to adjust? It’s a bit tricky to replicate the same glass rendering across different sessions.
The closest I can think of is you could load the transparency image as an overlay, which you can set the opacity for on the command line: -overlay.opacity 0.05
Then you still have to load a main image and deal with that. Unfortunately, it seems like selecting “hide main image” in volume mode hides everything… So you can either manually drag the opacity of the main image to zero in the gui to “hide” it. Or, if you want it all on the command line, load a main image made entirely of NaNs 
I did this
mrcalc mask.nii.gz NaN NaN -if nans.mif
mrview nans.mif -overlay.load glass_brain.nii.gz -overlay.opacity 0.05 -mode 3 -plane 1
and got this:
This may get more tricky if you have a more complicated vis setup with other overlays etc. but hopefully this helps.