Based on Digijam's suggestion, I made some balsamic vinegar 'caviar' sized spheres for a dinner party -- served them with a blue-cheese in a pear-chip "cup" (about 5 small spheres on the blue cheese, in the pear-chip cup). About 2/3 balsamic (*not* "tradizionale”, but just the "Napa Valley Naturals Grand Reserve" -- cheap, but I think pretty good), 1/3 simple syrup, gluco to 25g/l, standard algin bath of 5g/l.
Worked great -- perfect taste, and the 'explosion' of balsamic after the crisp pear and smooth Gorgonzola was perfect. Many of mine still stuck together (d'oh!) but it wasn't a big deal as enough didn't that it wasn't a huge problem.

I wonder if the problem might be the amount of sugar syrup. If you can't bag any xanthan it might be worth trying another thickener.
I also served a 'deconstruction' of the walnut and pear soup from the French Laundry cookbook, making "ravioli" out of sweetened pear puree, and serving them in a chinese soup spoon with warm lightly whipped walnut cream. I rather enjoyed it, but it was not as universally loved as the cheese & balsamic, alas. Some of our guests really loved it, some really didn't like it at all. Some rather rude things were said about it.
Ah, well.

I'm sure Ferran used to have the same problem (and sometimes still does)

. I have found guests tend to be more willing to try the softer textures - foams, spheres etc - when they're supporting something more solid, even crunchy. Gives then a safe point of reference.
Thanks for all the great suggestions and ideas! I was having my doubts about this technique, but I've now found at least one thing that works great with it and not so well at all without it.
jk

Haven't had a chance to test the strength of Gluco spheres yet, but if they stable and heat tolerant I reckon their best role will be when hidden within other textures. Spheres enrobed in chcolate, and caviars rolled into ganaches or even cakes. Kind of a special surprise supporting role, rather than the main event - like pop rocks in reverse.