Have to be honest with you Aaron, the Design map features are the probably the least understood feature for me right now. (Heh, and I even got decent result with the RainMaker script and v2.9 last week for some texture splatting work in Photoshop to edit into the final Attribute Map with the custom Climate I'm working on this week...Salinity..wow, that was an esoteric adventure).
Design Map painting is the next adventure.
The current workflow I'm investigating has involved editing a few mountain ranges into the Volga River Basin/Black Sea area for an RPG universe (Earthdawn, Fasa/RedBrick).
I'm working at a near 1km/px scale (although I actually have used my-scale/10 with the original vertical range in L3DT to get good results on certain passes/iterations, see below). I should cross post some of my work-in-progress to a new thread. Most of the editing and blending I've been doing with PS and Mudbox (mainly for the visualization possible with Mudbox, where you have PS like visibility layers you can pant on, and toggle visibility for). Oh, the Wacom Intuos 4 really helps bring out the best in those, also.
So, because I'm working close to a 1km scale (733m) I clipped out a lot of the sea floor depth, and would use the Adjustments> Curves function in PS to "push" (or stretch, I guess) the lowland detail into more of the image's 'range' (losing details in the upper altitudes). There is a "blend-if" option in Photoshop that let me apply this only to ranges above sea level(that was an interesting bit of math). This is now my "Res-Bumped" layer (I adapted the concept from Tom Patterson over @ shadedrelief.com). Then, the highland detail in the ResBumped layer get a custom smoothing pass, with a lot of brushwork, and then blended with a copy (on another layer) of the original DEM. See, with the raw ResBumped layer just blending with the original Dem (at about a 3:5 ratio) you get visually "noisy" artifacts in the upper altitudes. Once that has been smoothed out by the brush-work, you're basically projecting the original DEM onto a "bubbled" version of itself. This visual exaggeration of features that would normally dwarf our own personal human scale leads to "mountains that look like mountains", which is something some dead cartographer said which is now lodged in my head. (Thanks Eduard Imhof!)
Then, in Mudbox & PS, I setup a physical material with a red mask where I wanted the Mts to go. I used this to sculpt up the ridge-lines and peaks following the natural ridges on my DEM data in the other layer. Lots of slow brush work (I did some rough-outs of the same process in L3DT but the lack of brush-tip falloff, easy brush 'opacity' changes and layered visualization with the ability to export individual layers made more detailed work possible in the other tools).
What I found L3DT
essential for was adding realistic visual complexity to the forms that I had added in PS/Mudbox (which, by themselves looked doughy or lumpy compared to the DEM data). I think I generated 3-4 various peak and ridge maps from PS/Mudbox, and then (this is when it got exciting for me, no srsly) I would import those into L3DT as 512x512 or 1024/1024 Design Maps, and change the Wizard Setting over a few iterations to get the feel I wanted. Armed with these, it was back into Mudbox so I could blend them with a high degree of control, and then layer them onto the ResBumped+Dem version of the original HF.
Here's two shot with a very basic color-by-height&climate texture, rendered in the UDK/Unreal3 Engine as a 2017x207 heightfield, with 4 adaptable LOD levels which blend intelligently between each other (I know, right?). Screenshot is from the Editor, with Game-Mode & Real-Time effects setup and enabled (depth-of-field, atmospheric fall-off). The vertical range has been scaled up by an arbitrary amount based on my artistic sense.
In the first shot, the mountains on the left are DEM data, and the center are edited in. In the second, the Mts on the right are DEM and all the ridges in the center are edited in. You can see the difference, but they're visually similar, which helps the immersion and suspension of disbelief.
I would NOT have been able to do this without L3DT.