I am not familiar with Hardiplank, but at only 3/4" thickness, and 24 foot lengths, you are definitely going to need some sort of reinforcement to keep the sides from bowing out, even if 2-3 inches are buried. I would also suggest some reinforcement (maybe only one piece of reinforcement in the middle) on the 4 foot side, as well.

The plan of filling the box with compost sounds good.

I don't think it'll work. It is only meant to be installed in a very precise way and it will even deteriorate if it's installed too close to the ground, let alone actually touching it. Contractors have to store it inside under cover until it is put on the house.

I have pieces of Trex decking purchased 10 years ago that are almost like new. Like Hardi, it's a composite board made of sawdust and recycled plastic, absolutely termite proof. It comes in 12 foot lengths, cuts and screws like wood, lasts forever. Quite heavy, the only drawback.

I have some plastic corner connectors (from Gardener's Supply 15 years ago), which work well with the decking planks. I can put up or take down a bed in 10 minutes. I've screwed pipe-holder doolies (pipe strips?) onto the outsides of some of the planks, which hold wire arches over the bed to support plastic or row cover.

I'm trying this as well right now, had some laying around in my garage. Definitely had to drive some extra stakes along the long side.