So, anyway, I've done a few more modifications to the Accidental Engine. Previously, I was storing tilesets as specially constructed bitmaps with an alpha channel rendering portions of the map transparent for use in the various transitional configurations. I've changed it to use simple RGB bitmaps with the alpha channel applied at render time using multi-texturing. This way, I can construct my tilesets not simply as 8x8 banks of tiles, but as one large tile that covers an area of 8x8 in-game tiles. The effect is the reduction of the obvious appearance of tile-based-ness. Instead of graphics repeating every tile, the repetition is spread out over an 8x8 area of tiles. The difference is subtle yet significant.
It also reduces the size of the tilesets, since I don't have to store alpha information per tileset. On the downside, doing anything but a smooth blend for the transition (such as special border graphics and such) is out of the question.