I wanted to achieve an Orton effect for some clips, so if anyone else is interested in this, here's a relatively simple way to get something close to that in Kdenlive. You will need at least 3 video tracks.
1) Place the same clip in three tracks, all aligned on top of each other.
2) In the lowest track, place a "levels" effect on the clip, and set the "White output" slider to around 500 (it can be tweaked later).
3) in the next track up, place a "screen" transition and stretch it so it takes the full width of the clip.
4) in the topmost track, place a "multiply" transition, also making sure it is the full width of the clip.
5) in the topmost track, also place an "IIR blur" effect and set the amount to around 0.500 and the type to around 0.100 (I'm not sure what the type parameter does, but it's pretty subtle).
The output of all of this will have an ethereal, Orton-like quality. It's similar to the "glow" effect but more subtle and slightly increasing of color saturation and contrast. You can tweak the look with the "white output" and the blur radius.
Unfortunately, because of the way Kdenlive does compositing, and because it doesn't really let you bounce or redirect tracks, if you want to do anything else with the result, like even just crossfade it with another clip, you must first save this as a project and then load that project as a clip in another project -- a little cumbersome, but it works.