A diffuser will not do anything to help reflections. In fact, if it is a large diffuser, it may even make it worse if you are not careful. If the ceiling is actually transparent glass, even if it is tinted, you will not be able to bounce from it since most of the light will go through it. If it does reflect, it will be just like the harsh light that left the flash except at a different angle since the glass would act like a mirror.
I would shoot with direct flash.
The longer the lens, the narrower the angle of view and the easier it is to control reflections. With a wide angle lens you may be looking at 20 feet of glass in the background. With a telephoto, you may be looking at 5 feet of glass in the background. The telephoto reduces the chances of reflection. Of course, you may not be able to back up far enough to use it effectively or your flash may not be powerful enough to use it, but you will be able to control reflections better.
The flashlight trick is a good one. It helps you see where your flash reflection would be. However, it may not be convenient all the time. It may be easier just to make sure you are at an angle to the glass. The greater the angle, the less likely you are to have a reflection of your flash. The closer you are to straight on to the glass, the greater your likelihood of getting your flash's reflection.
If the building is actually a pyramid with the glass walls at an angle to the floor, then you may not have any problems at all. You may get some light patterns due to light from the flash reflecting on to the floor or your subjects, but if you play around with the direction, you may be able to control that.
