There is no USABDA Regional in Los Angeles in June. You must be looking at last year's web site when the event was in southern California. This year's USA Dance Southwest Regionals (previously called USABDA Western Regionals) is in San Jose on June 11, 2005.
I am on the organizing committee for this event. We do not allow costumes in any syllabus event at all. Our information page is at www.usabda-norcal.org/regionals . If you click down into "General Information" and look at the section on "Syllabus Restrictions," you will see the following statement:
"Costumes are not allowed in any syllabus category, regardless of age group."
Regarding your comment that the new dress code rules in the 2005-2006 rulle book on the National web site seem all the same, I'll reply that they are and they aren't

They certainly will seem all the same to someone who has already been following the "no costumes in syllabus" rules for the past few years. They are just more clear and more specifically spelled out now. At one point they were trying to make a rule that ladies in Adult Syllabus couldn't wear heels higher than 2", but that got removed (thank god). Another rule that showed up earlier this year and then disappeared was that men in syllabus Latin/Rhythm could only wear white shirts. Now they can wear white, black, or midnight blue.
There are a few new details that people need to keep track of, though. For instance, there never has been a rule as to what color shirts men could wear in syllabus Standard. Now it's white only (black or midnight blue is also allowed in Smooth, but not Standard). There never used to be any restrictions on what materials a lady's syllabus dress could be made out of, but now certain types of materials -- including metallic and glittery -- are not allowed in syllabus.
Also, and most significantly, it used to be that the syllabus rules applied to ALL age group events. Now, "no costumes" syllabus dress applies to all age groups EXCEPT when specific Senior syllabus events are offered. Costumes are allowed in Senior age-group syllabus events. (By the way, at Southwest Regionals we don't offer separate Senior syllabus events, just Adult-age-group events, so therefore we don't allow costumes in any syllabus event at our particular competition.)
Any other questions? Let me know!!