A thought ... you may be able to fabricate something (a small cup) to place down in the spark plug hole (bring the piston to TDC), which would catch any shavings from the tap ... then once you've installed the heli-coil, you could remove the fabricated "catch" which should contain most (if not all) of the metal shavings. I say this because even if the heli-coil is magnetic, the metal shavings from tapping the head will not be (as the head is aluminum) ... and those metal shavings will not do any good for piston rings and valves.

Hang on, we're goin for a ride!