You need to have 400 sq in of blaze orange minimum above the waist and visible from all sides. If you have a tiny minimum vest and they cover the back of it with a backpack then if most likely will remove your minimum visible area to under 400 sq in and may also remove the visible from all sides component as well. On the other hand if you wear a full blaze orange full length coat then the backpack would not reduce your minimum visible area to under 400 sq in and the material at your upper shoulders, back of your arms, lower part of your coat would be visible still, therefore not removing the visible from all sides component of the regulations.
If you have a really nice camo backpack that you really like, you can still use it, but depending on your blaze orange garment underneath you may want to either clip some blaze orange on to your pack or get a cover for it.
I actually want to get a frame pack for backpacking and I would rather it be camo then blaze orange. I will be taking this pack and finding some blaze orange material and affixing it to the back of the large pack with safety pins or clips of some sort for added safety.
Good luck in your hunt for a pack, but I understand that blaze orange ones are not the most numerous.