I don't think it's a good idea, since, even with the door open, it's still in a small space, with very close walls on three sides of the sub. Low frequencies are non-directional, but the small space will reinforce those low frequencies, and you'll have booming, muddy bass that will probably not sound very good. Try it out; who knows -- it might sound okay to you, but I think most would still suggest you place the bass out in the open, if possible.
cheers,
supervij