Theoretically, its not safe because you are connecting a device with insulation designed for 100V RMS to a 240V RMS circuit, and if anything fails on the neutral side of the circuit before the Variac, the insulation to case or exterior metalwork has to withstand the full supply voltage. In practice, under the direct control and supervision of a competent person, its fine for bench testing and if the knob is locked, for general use by said person *AFTER* they have satisfied themselves that the equipment's insulation, creepage distances, clearances etc. are adequate for 240V - beware of cheap & nasty rocker type power switches!
If anyone else in the household without an EE/EEE background is going to use it, either convert it to 240V operation (if its possible to safely do so) or get it a voltage conversion transformer, preferably *NOT* an autotransformer.