They're living Metal, EMP doesn't/shouldn't affect them like other Machines and AI. Though if it did do anything to them, it would only cause them to pause for a handful of seconds as the small amount of damage is repaired.
An interesting thought indeed.
However, out of sheer curiosity, how good is something like that if said Necron lands or is hit by an EMP mine, which Rakai'Thwei also happens to utilize in his hunts?
"The Necrontyr originally invented living metal for use in constructing their ships. It is a metallic substance that appears to grow and heal as if it were part of a living organism. It is most often associated with the skin of the C'tan as well as the bodies of the Necrons and their amazing regenerative abilities.
The hull of all Necron vehicles, including the Necron Monolith and the Necron Pylon, is also made of this living metal, where its adaptive and regerative capabilities render powerful weapons such as Meltaguns, Multi-meltas and Power Fists almost obsolete.
This deadly testament to Necron power has inspired the infamous Tau saying "Living Armour Happens""