So I'm using the backpack script from here, but it doesn't work.
http://hce.halomaps.org/index.cfm?fid=2802
In sapien it says scripts compiled correctly, so no debug.
When I build my map, no backpack.

I'm using the Halo 3 weapons, and Bourrin's Mark V.

I've never used it, but I'm assuming it either uses model permutation or an object attach script. Either way, it looks like the script was set up using the stock halo weapons. So if you want to use it, you will either have to use the biped and weapons it was made for, or make/modify the script to work with you biped weapons. Although, depending how the script is used, you might have to use the biped it came with, or modify the one you want so that it will work.

you need to make a biped with permutations of the actual weapon models on its back.

Aww, shucks...
It would've looked really cool on the Mark V...