OK well, I still don't see how a 1.5 inch bore hole in one side the tub is going to result in any great air exchange, let alone any drying of the sump, even with a fan going 7x24.
Of course there must be some venting of the tub; otherwise it would explode from the pressure of heated air inside it. I alluded to that with the detergent drawer comment. As far as I know with the Miele, the detergent tube is the ONLY venting into the tub. Unless I missed something last time I had mine opened up. Certainly that's where the suds comes out if perchance too much suds have been created during a wash cycle.
The Neptune actually has a vented door; since it's a tilted tub there's no need to hermetically seal the door, although it does a pretty good job of keeping excess suds contained. The vents are at the top of the inner door panel, with a big oval orifice at the bottom of the inner door that allows some air movement out of the drum. Although I've never had a mold/odor problem inside the tub, lately I've taken to leaving the detergent drawer lid open all the time, even during washes, since it helps to dry out the detergent drawer where a light coating of black mold around the fabric softener area does eventually make its presence known. The detergent lid is vented as well, but apparently this isn't enough to keep down the damp.
But I still don't think a fan would do much if anything.