Ookay. I did a lot of research, and to answer my own question:
The Internal SD can NOT be formatted, because it's not a separate device, but merely a directory under /data. /data/media, to be precise. But it can be ERASED/wiped.
Wiping /data now no longer format the /data partition; rather, it simply deletes everything under /data except /data/media.
CWM (or other recovery) should have a separate command to erase/wipe the /data/media directory.
And to find out what filesystems are supported, one should do cat /proc/filesystems
Thanks for reading! I hope this information can be useful to you, too.