You can install Mobi2Mobi (non-gui), whose metadata manipulation is really the key to all of it, from scratch using the
instructions on the wiki. It's a bit of a hassle to get properly working at first, but once it's set up, it's pretty easy to use.
After that, just set the ASIN using the Terminal (
wiki instructions for using Mobi2Mobi on a Mac) which you can alias or maybe even make an AppleScript to help speed up batch processing, and sideload your tweaked books onto the iDevice in the usual manner, and you should be good to go.
ETA: You'll want to be following the second set of instructions in the Mobi2Mobi usage guide, on setting the metadata using the --exthtype option.
The EXTH.pm file that comes in the Mobi2Mobi folder gives you a list of the types that you can use and what to call them in the command.