As far as I understand (I have an N800, not a 770), the main concern with 'becomeroot' is that the user (you in this case) might delete a critical file or erase the built-in flash. Installing becomeroot, moving .wad file around then logging out of the root account isn't any more dangerous (AFAIK) than developing a complicated installer (that's running as root) to do the same thing.
Someone who has a 770 can correct me if I'm missing something...

-John