eva

If it's an "all styles welcome" seminar and the seminar announcement doesn't explicitly say "hakama for dan grades only" then wear whatever you feel comfortable with.
I wouldn't judge people's abilities by what they wear, but I know that some people do make assumptions. So if you are not sure about your ukemi skills make sure the colour of your belt can be seen.
One thing I really hate that happens a lot at some seminars is that if you wear a hakama you get the chance to practise with a lot of advanced people and if you don't wear a hakama and/or BB you end up practising only with relatively new beginners which can make life difficult if it's a seminar of a different style than what you usually practise. On the other hand if you wear a hakama some first kyus etc (who aren't allowed to wear a hakama) might try to test your abilities...
As other people have said: if in doubt ask!

Eva