I came upon this old post and did a little more searching, had some results to share.

From this PDF (p.24) for AQUALONŽ.

The effect of trivalent cations on CMC solutions can be controlled and used to advantage where gelation is desired. Gels of varying texture can be produced by careful addition of certain salts of trivalent metals, such as aluminum. Gradual release of aluminum ions to a CMC solution will result in uniform crosslinking of the polymer molecules between carboxymethyl groups. Gradual release of aluminum ions can be accomplished by using a slowly soluble aluminum salt such as monobasic aluminum acetate, AlOH (C2H3O2)2; soluble salts such as aluminum sulfate, Al2 (SO4)3, in combination with appropriate chelating agents; or insoluble salts such as dihydroxyaluminum sodium carbonate (DASC), Al(OH)2OCOONa, followed by in situ formation of the soluble acid form of DASC.

Properties of CMC gels depend on many factors. In general, the stiffness of a CMC gel increases with:

- An increase in CMC concentration
- An increase in CMC molecular weight
- An increase in the concentration of trivalent metal ion.
- A decrease in solution pH.

Techniques for producing CMC gels by crosslinking with trivalent metals are discussed in more detail in Aqualon Bulletin VC-521 and Bulletin VC-522.

I tried looking for these bulletins, to no avail. But at any rate, there appear to be methods for crosslinking / hardening CMC gums. The nice thing about this gum is that it's easy to work with and doesn't cold set like gelatin; so in some sense it might be more convenient to work with. Who knows though.. just putting info in the right places in hopes that someday it'll be useful to someone.