You may want to try WIT to extract and recompile your .iso (
http://wit.wiimm.de/)
On Windows these commands will allow you to do what you want in WIT:
WIT can be downloaded here
http://wit.wiimm.de/. The documentation is also pretty thorough.
After installing, run this command in wit-console.bat with the proper values (this may be a different file if you aren't using Windows).
wit extract BrawlISOName.iso workdir.d
^This creates a filesystem in workdir.d from a BrawlISOName.iso
Then edit the filesystem with the files you want to add/replace
Then run this command in wit-console.bat
wit copy workdir.d BrawlISOName.iso -o
^This creates BrawlISOName.iso from the files in workdir.d. It may take 5-15 minutes depending on your computer's specs