The maintenance message does not go through the bbcode filter, it doesn't go through any for that matter so you could use HTML. The reason it doesn't is probably in case you're making file/database changes in which case the required files for bbcode may not be available.
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):
Win, Linux/1.3.34/4.0.26, 5.0.19/4.3.10, 5.1.2, 8.1.4(PostgresSQL)/188.8.131.52, CVS
Joined: Aug 18, 2004