Some of the fixtures at the link are for MR16 lamps which have a variety of wattages and beam angles available to help control spill light. SPJWM-1600 Reflector

Here is a link that might be useful: suggestion

Just be sure that you don't aim anything directly at your window. You can use a variey to different types of lighting. Graze up from the bottom with a flood light aimed up placed just in front of the window, angle so the light is grazing and not spot lighintg. You can also mount something above the windows and aim down to graze the window. You have to consider what these will look like from outside your house as well. The sign lighter in the first comment is a great fixture, just don't aim it so it shines directly in the window. Any good quality flood light that allows the bulb to be recessed into a hood will work fine in both applications. 120 volt will work just fine. Select something based on the distance that the light has to travel from it's mounting position to reach the window. Remember, what you are trying to do is create light outside the window, not shine directly into it. If you decide to use 12 volt fixtures, use floods, not spots. Make sure anything mounted in the ground has a watertight cover over the bulb. If done correctly, this technique will create a nice wash of light on that side of your house as well.

Thanks for all the information. I'm thinking that I need to do some hands-on experimentation by buying several different narrow floods to see what might work best. If I'm not too close, and can direct a lot of light that's not too wide a beam, I think I'll accomplish what I want.

I don't have easy access- given the way the house is built into a hillside, it's more like a second story where the windows are located. There's a tree directly across from them, about 30 ft away. If I can get to the proper height from there, I'm not sure if the distance will be too great. I don't want a lot of light to be coming in alongside the stained glass windows at the same time.

I've heard something about lights with beams that can be framed and focused on a particular area- assume this would not be an inexpensive solution though. If someone can direct me to a website that shows coverage area (like I want to cover about an 8 ft x 8 ft area with light) from various distances with various intensities, for example, that might help.

My best viewing is from 9 a.m.- 9:30 a.m. with nearly direct sunlight. When the leaves are gone and the sun is a little lower, it should be even better. I don't expect to achieve anything spectacular at night, but a moderate illumination would be better than a totally dark viewing of the windows.

Perhaps you should look at 1 or 2 12 volt MR16 floodlights mounted in the tree.
A GE 50m16NSP/IR lamp would deliver about 16 footcandles on your space, with a beam diameter of approx. 7'.
If one lamp is not enough, install 2 ,aimed to the same spot.
12 volt fixtures are easily install in trees, but , of course, the downside is relamping at that height.
Keep in mind that most good quality bulbs are rated 4-5000 hours.Probably 3 to 4 years use.
I would install a good quality fixture, like a Hadco BL5016 or it's equivalent,using a tree mount attachment.

Try these , I just used some to baclight some translucent printed acrylic sheets that were 4' X 10"

http://www.1000bulbs.com/images/PDF/NTF3218-3227-3239-3240-Specsheet.pdf

This was for an indoor application but maybe a search for a waterproof version will produce some results

Cheers Roy