It sounds bad that you should need to have html turned on just for the marquee-tag. But I haven't got a clue as to how the html version of it works, and I don't personally like the use of it either. Because of this I will quite probably not include a marquee-tag in the hack. Although nothing hinders others from adding it.
From what I can find on the marquee-tag, it is the direction and behaviour attributes that you would be interested in. If you don't know how to do regular expressions you would probably have to do one start tag for each type. Don't know how a all-in-one tag would look anyway, maybe something like: [marquee:direction,alternate]
Where direction can be up, down, left or right, and the optional alternate sets if the scroll should go back and forth.
Add these two (as "Perl Regexp /") and you should have a marquee tag that works as mentioned above.
code:
Markup: \[marquee(:(up|down|left|right)(,(alternate))?)?\]
HTML: <marquee direction="\\2" behavior="\\4">
Markup: \[\/marquee\]
HTML: </marquee>
