Pretty sure that there is no way to do this, but I've heard rumors (maybe from older versions of the wardrobe) that it's possible to layer a background over the HA base so it doesn't appear in the final HA. Can anybody fill me in on if there is any truth to this?

You can't do that because backdrops are drawn with a HA shaped hole for the HA base, like this

The only way to get rid of you base would be to use the Vanishing Cream, which I see you're already wearing :)

That was what I was worried about. I was hoping that it might be drawn as a full image, and the gap appeared because of layering. Thanks for the help :)

I'm pretty sure it's not possible to do, but I've used backgrounds to hide some awkward pieces of clothing I didn't like on the HA. Maybe that's what you're talking about? If it's not, I'm not sure. Sorry and good luck! :D