Below is the URL for a video showing how to employ Golden (my favorite brand in art supplies) textured gels:
http://www.goldenpaints.com/VIDEO/library.php?ID=52
Better modelers no doubt can have superior techniques, but I think this stuff works pretty well – I'd think there's promise to at least equal the pigment/plaster/resin/sand brews encouraged by MIG or AK. And $15 would get you a lifetime supply.
PS: I've continued to futz with the basic mixture above varying the colors and viscosity. The other efforts were not as successful as the first for wet mud. However, I made a brew with matte medium and then over painted it with Ranger “Distress Crackle Paint” to see whether something like this might not work for thick dried mud. Keep in mind these are one-off medical experiments and could presumably be improved with some effort. (I would certainly not have put as much on for dry mud.) But the crackle stuff does crackle the surface okay, and evokes memories of my times in Minnesota where a pounding rain could turn an area into a swamp and the following 99 degree sun dries it out – and it's crackled. Ranger paint was pretty pricey but checking the Blick web site there are craft equivalents available for $2 and redoubtable Golden makes a “crackle gel” for $11 – for enough to crackle Idaho.
Eric