Hey, volunteer here!
With Itemsadder you can add a custom texture to the backpack, instead of a re-colored head.
You can read about the compatibility for the "EpicBackpacks" plugin here if you own itemsadder. however no, I do not think that you can make it look like they are actually wearing the backpack with the plugin you mentioned, it's more of a right click to open in hand type of thing.
You can see how it works in this review, however its in spanish.
https://www.youtube.com/watch?v=4tykRsEw3No
However perhaps with some commands and plugin magic you could make what you want happen with cosmeticscore, it's not anything that's officially implemented though, so don't count on it.
Would be a good suggestion though.
