My guess would be overlay or multiply. Will you be printing only black onto the glass or a white or colored pigment also? If it's a traditional glass negative, multiply or darken would be the mode.
Depends on where you lightsource is on the glass too I suppose.
I will be printing full range images... using a traditional inkjet ... so i would assume that white would end up transparent.
Multiply/Darken makes sense... so I suppose varying the opacity of each layer would be necessary to keep from building up too much darkness.
I do a lot of layering in my work in PS in general, and want to experiement with using glass panes, in box ...not sure if there will be any special lighting other than what comes from the front in. Probably the backmost image will be on regular paper, alum, or wood or something solid.