I've used bone and FWI on acoustic guitars with USTs without any problems whatsoever. And the unamplified tone from an organic saddle is better than from inorganic materials.

However, I've dispensed with USTs altogether now, and I use K&K Pure Western under-bridgeplate transducers. A far superior-sounding solution without any interference with saddle/bridge contact, and with none of the 'quack' that USTs always seem to produce (and before anyone starts squeaking about feedback, I have no more problem with feedback now than I had previously with USTs, and if I do get feedback, careful EQ-ing and use of the notch-filter takes care of it).

IMHO. YMMV.