Can you find a picture of the look you are going for? I'm no help because I can't ever recall seeing sconces mounted on window trim.

unfortunately i can't find a picture.

This is aokat15's kitchen, have you seen it? Not sure if this is a similar look but I don't think it's really mounted on the trim.

yes, i've seen this pretty kitchen :)

this trim is a little wide and i'm now worried that, in order to mount sconces, the trim needs to be flat like this and this style might be too contemporary for our room? any thoughts?

In that kitchen, I don't think there's really trim between the windows where there are the sconces, or if there is I think it's different from what is on the sides.

Not that I know anything about syle or authenticity - just so you know, but IMHO whether it looked contemporary or not would depend on the light. I don't know if this might be an option but they make blocks for mounting lights on the outside of houses that have siding so the lights can be attached to a flat surface.

HTH, good luck

Does your trim have a deeper band on each side (same thickness on the part that goes against the window and the perimeter), or does it taper from the part against the window to the perimeter?

I would probably interrupt the molding and place a block that is the same thickness as the thickest part of the molding underneath the backplate of the sconce. I would make it slightly larger than the backplate so it clearly appeared that the sconce "rested" on something, but I would not emphasize it more than that.

We have a "back band" on the outer perimeter of the trim if that helps answer your question.
I'm having a hard time imagining your suggestion not looking slap dash. Should I just do a flat trim instead?
I've yet to choose sconces, but was thinking along the lines of a library style.

I would use flat with the same back band, then.

Whatever you chose for a sconce make sure you can open the two-the-counter cabinet adequately.