Alan, are the ferrite toroids used to filter the output of pc power
supplies or the ac input of microwave ovens useful for other applications?

The PC PSU filter toroids seem to be optimized for the 25KHz -85KHz
range so any applications you have for this frequency range could use
them. Similarly, the AC input to the oven is probably 50/60HZ so those
toroids would be better at the lower audio frequencies.

Or is the ferrite material lossy like the small ferrite beads used for emi
reduction on wires?

Not quite sure about beads - ordinary ferrite material has fairly
constant permeability up to a frequency determined by the ferrite mix,
then drops off. Beads tend to have an impedance peak at some desired
frequency range - e.g. 100-200MHz
Alan