Can I use calcium reactor media as a substrate alternative to rock rubble?

well calcium reactor media is just argonite sand so i guess you could use that. But IMO i always use live sand when starting any fowlr or reef tank.

Would crushed coral work? I would think it should be coarse enough...

What do you mean by using rock rubble as substrate? Like as your said bed.

If so then no, bad idea. Crushed coral is also a bad idea in reef tanks.

Crud accumulates in crushed coral than starts to rot.

If it is the stuff that looks like rubble, yes. I suppose it might be cheaper to buy that "dead", dry media than LR.