Here's my guess:
1) Copy before you do anything.
2) Soak in clean, distilled water until it comes loose on its own. Add some PhotoFlo if necessary.
3) Dry and flatten as you would any other print, except being extra careful.
4) Once you have the photo safe, make at least two more duplicates. Make one traditional duplicate and one digital duplicate.
Try to remove the picture and the glass from the frame as one unit and soak the glass and photo until they come apart without peeling or pulling. Hopefully, once the paper soaks through, they'll just "float" apart on their own.
If you can't get the glass and the photo out as one unit, consider sacrificing the frame. Pull it apart, break it or cut it instead of risking shear or pulling force on the photo.
That's all I can think of.