Okay I'm doing a 2 tier cake (8" and 12" square) and currently working on the bottom tier.
I bake 2 cakes per tier, and slice both in half with my agbay so I get 4 layers of cake and 3 of filling).

Anyway, I stupidly didn't support one of the cakes for my 12" bottom and it is now in 6 or so pieces. It's still together in the square shape, but it's cracked all over the place. I'm thinking of trying to layer it and then trying to fill it so the filling kinda 'glues' it together. Will that work?

I do have time to bake another one, it just means going to the shops tomorrow to get more ingredients and baking it later in the day (in between taking my son to kindy gym, going to a house inspection and then having family over for dinner).

Can I save this cake or should I just send this cake to the Cake Heaven (ie: Cakepop Land)?

I'd make a new cake. IMHO, once the integrity of the cake structure is compromised, no amount of "glue" will hold it in the proper shape. Especially if you're stacking another on top!