I use coconut coir for my composting worms and it works great. I keep all my night crawlers in it also and they have never died like they did on peat moss. Have 2-3 dozen I have had in there now for over 7 months feeding them on top every other week or so.

I also use it for my Pacific Chorus frogs in a planted terrarium and they love it. It does stick to them a little but they get over it. I have never seen them eat more than tiny bits of it and they have no problems spitting it out of they get a mouthful. It was the highest recommended product I found for substrate on most sites I visited before setting up their tank, but it was NOT recommended for use with non terrestrial species.

The main risks of impaction are if the terrestrial frog/reptile is small and/or the substrate is too dry. The key is to keep it moist and not let it become a dehydration problem if swallowed dry.

The only real problem I see with your night crawlers may be that they like to burrow and form tunnels for homes. If you are not using it for breeding them I doubt it will matter much.

I feed my garter snake night crawlers kept in it and have for 2 years with no problems, Not an amphibian I know but impaction can be a major problem in them also.