Not sure how useful this will be, but I just added the ability to do custom slope/aspect/elevation shading to caltopo.com, and wanted to drop a note about how to bring it into MM.
First, read
the documentation as there's no slick UI.
Then, go to caltopo.com and set up custom shading using the layers control in the top right. Pan the map a hair to update the URL hash.
Copy the end of the URL, after sc_. Note that this is the same as what you entered into the text box, but with the space replaced with the letter c and line breaks replaced with p. Sneaky . . .
Once you get used to things, you can just create the shading code by hand and skip to the following:
Under Configuration / Map Settings, enter a new custom map layer. For the URL, use the following:
http://caltopo.com/resource/imagery/shading/config_str_you_copied_from_url/${z}/${x}/${y}.png
max zoom 16, opacity 30, type OSM.
That's it. So far serving custom tiles on the fly hasn't posed an issue for my server, but please be mindful of the load you're placing on it when caching them for an offline mission.
