Apparently Costello it is very possible, and isn't it one of the features for Wiinja?
Modthatcube is either down, or has moved or something, but they had info on it.
Apparently you use this tool - viper_multi_boot_iso_creator_v1.0b-loonycube.rar
A google search returned some info on it, but like I said modthatcube is down or something, but the cached link works(sorta).
http://72.14.203.104/search?q=cache:MpTDqg...clnk&cd=6&gl=us
http://gcfaq.gc.funpic.de/mambo/index.php?...selectcat&cat=1
seems a 1.3b exists also, should try to secure that one too.