Fiction Foister in Fairbanks writes...
If you've already published a novel, is it inappropriate to apply for an MFA?
No, not inappropriate. But you might run into some resistance with the committee. They might believe that you won't listen to what they have to say. The bottom line seems to be: You want the time and the community. Go for it, if you think it's the right thing to do.
As for disclosure: I'm assuming there's no question on the application that reads: "Have you published a novel before?" And if not, I wouldn't mention it. It might work for you in some circumstances, but overall I think it will work against you.