It is it possible to safely use a 110V variac (variable transformer) at 230V?
No, the core will saturate and (if you're lucky) blow a fuse. 

If your 230VAC happens to be (say) 400Hz, the answer might be different. 

Wikipedia has a very nice animated image showing the effect of too low frequency. Too high voltage has the same effect. The value of (maximum primary voltage)/frequency is a constant for a given transformer design). 



As you can see, the magnetizing current spikes up twice per cycle and the resistive heating losses increase with the current squared.