I tried it but didn't get very far. I used Windsor pastels but I wasn't able to get the powder to adhere to the plate surface. Maybe japanned metal works better than trophy plate?
Pastels work for very small effects such as cheeks, etc. To color large parts of the plates it was common for photographers to use diluted oil paints or glass tinting paints. Even the old Kodak color sets for paper prints should work.
If you want to use pastels I would first give the plate a thin coat of varnish. This is done the same way as you would normally varnish a plate, but pour off a little faster then normal. The plate will have a dull finish if it was done right, instead of the high gloss you normally get. Once it is completely cured then the pastels should stick much better. After you are done then give the plate another coat of varnish. Remember to color it more bold then you think you should. The varnish will mute the colors down quite a bit when you put the second coat on.
I have done this a couple of times with good results.
Pastel chalk works great for large areas; I've done entire blue skies with it. All original manuals only give chalk powder as the coloring for dags, ambros, and tins.
This from the Silver Sunbeam:
"For touching up daguerreotypes, ambrotypes, melainotypes, and ferrotypes, colors in very fine powder are employed. These are laid on the appropriate parts, shaded off so that no sharp edges exist, and afterward the excess is blown off with an India-rubber blower...."
Just gotta learn the technique; most guys try it once and then say, "It doesn't stick.."
Take a small stiff brush and just rub and rub the powder into the image the best you can, blow off the excess and with your finger rub off the areas you went "over the line." Yes, you can rub all the color off at this point. Once the alcohol of the varnish hits it, it just liquifies the powder and it is there. Don't leave a big ol' bunch of powder over the image thinking the thickness of the powder layer will work, it will just streak color all across the image. Just rub the powder in with the brush. For a large area on say a half plate, I may rub circles with that brush for 5 minutes or more getting the color like I want.