Unsure why foam sauce doesn't hold
I suspect your first problem is using the iSi whipper.  It does create foams, particularly whipped cream, but both Khymos and Texturas advise using an immersion blender or electric eggbeaters in a broad vessel (so there is room for the bubbles to pile up without interfering with your making more).  Besides, xanthan is shear-thinning, meaning that it is viscous only while at rest, becoming fluid when stirred or sprayed.

The second possible problem: while xanthan is used in some air recipes and is used to stabilize whipped cream and mousses, I find it is more effective as a thickener than a foaming agent.  Next time you make a foam, try using lecithin.  Lecithin is mainly used as an emulsifier, but it also dramatically alters surface tension, making bubbles more stable.  Very little lecithin is needed to keep foams stable, though the exact amount will depend on the proportion of oil and water.  For a fat-free sauce like the one you describe, I would suggest trying 0.5% lecithin by weight (i.e., for 200g liquid, use 1g lecithin).

If you really want to use the xanthan, you can try adding methyl cellulose to it.  I saw a recipe for an eggwhite-free "marshmallow foam" that called for 1.5g xanthan, 1.5g methyl cellulose, and 250ml water, plus vanilla, salt, and powdered sugar.