If you mean lightning as in discharges during storms, then the difference between a 750V DC tramway conductor and a 25kV (or even 50 kV) OLE system is lost in the noise of an 100MV cloud strike. As far as local flashovers are concerned, a 1500VDC OLE (when they existed here) for a given traffic density, would have a far higher current capability, liable to cause more damage and lifting of local earth potentials than a 25kV ac OLE which although rated for a longer section, assumes a current about 3% of the equivalent DC demand per train in section. On that basis, the clearance for voltage protection alone would still only need to be less than 50mm*, (witness the deployment of 6.25kV ac on established 1500VDC on the GE suburban lines in the '60s. The rest of the clearance as you say would be the kinetic envelope of live parts.In the current standards the real killer is that the default gap is sized not to transmit lightening to structures (+ allowance for upward movement etc.) rather than just the rail supply voltage.
* for 41kV max peak voltages