Braking by lunar projectiles?

I think a lot of the delta-vee necessary to bring asteroid material to
near-earth space is to get rid of the relative velocity between it and
earth. In other words, just getting an intersecting trajectory is (I
think) a lot cheaper. So it would be good if you could do some
equivalent of aerobraking.

I propose such an equivalent: launch stuff from a coil gun on the moon,
and have it hit the approaching asteroid. You could have the
projectiles be ultra-cheap spacecraft with limited maneuverability,
powered by metal-air batteries and having one in twenty be expected to
fail.