THB didn't have a huge advertising budget, but I have managed to find a couple of ads in the August 1994 edition of Powerplay. They showcase the great writing and atmosphere rather than the graphics.
I also found a cool postcard
in the November 1994 issue of ASM.
Finally, I stumbled across a very rare sticker
Let me know at (ecamber at thehiddenbelow.com) if you know of any others!