Can mobs spawn on glass?
I seem to recall at some point that a glass floor was not a candidate for mob spawning (which would be nice given that one cannot place torches on glass). However, I can't seem to confirm that information at the moment... is ittrue? (i.e. if I have a glass roof, and a glass loft, do I need to worry about somehow keeping it lit)?
Mobs don't spawn on glass, ice, half-slabs, and stairs. Your loft should be safe as per the minecraft wiki.
Spiders can climb glass walls, so you may want to add a small overhang or a row of ladders if there's an entrance.