Ah, great that you've been unbanned!
Well no one can post anything related to work, really, regardless of their history. However, it's generally less of a problem to post about work in a comment in a related thread, although I imagine if people tried to game the system they would be pounced upon rapidly.
And I can actually understand their reasoning. On Metafilter there is a certain ambient fear of "astroturf" efforts, big companies using paid-for shills to produce the illusion of there being a grassroots movement supporting and promoting them. Some time back, you see, there was a suspiciously enthusiastic post lauding McGriddles breakfast sandwiches, and not long before that a post announcing the refreshing beverage Pepsi Blue. (Pepsi Blue has since passed into Metafilter slang as referring to a blatantly commercial front-page post.)
On other sites too, there are growing suspicions, fueled by a few anonymous admissions, that there are entire companies devoted to hiring people to become members of forums, having them work towards becoming a trusted member of the community, all so that eventually they can hawk a client's goods without being suspected of being a plant. They take great lengths to avoid detection, and the nature of the internet is that a sufficiently motivated and knowledgeable astroturfer can probably go undetected indefinitely. This realization has made many people there understandably skittish when it comes to posts that look even slightly like an ad.
My suggestion is, if another thread comes up, in time, posted by someone else, it's probably okay to mention the contest in the comments, but only if you put in a disclaimer saying you work for Legend Entertainment. Even then, though, considering the prior event, it might not be acceptable. The only one could could tell for sure is matthowie, owner of the site.
Note, by the way, that Metafilter does provide some "official" banner advertising that is shown to guest visitors, which are likely the great majority of MeFi readers. They are not free, of course, but I believe they are not expensive. One would probably contact matthowie to see about placing one. (They used to have TextAds, which were very inexpensive, but those seem to have vanished, alas.)