No, I meant an ADDITIONAL sprite to every group of sprites sharing the same pose/size. If you want your billowing cape, then you add two "extra bits", aligned to the "cape" pixel on the colormap. The engine sees the pixel, sees the fact the creature is wearing a cape, and draws two extra "cape" bits as determined by a description file - one behind the creature, one in front of it, both aligned to the pixel and likely textured/colored according to whatever material the cape is made from. With a complicated-sounding, but actually rather straightforward system like this, you only need to draw special clothing sprites for each creature that has a nonstandard size or pose. That billowing cape would be largely the same for most humanoids of similar size. Armor doesn't have to be all sprites - you can have the same outline for a jacket and a chainmail, but you will use different texture for them. If you want custom-looking armor, make a sprite for it, but only then. Plate armor can just have a shiny look with shoulderpads added.
Then again, you can always go with Total Annihilation's approach. Take a single sprite-resolution camera and a lot of simple 3D models. When you need to change a critter's looks, assemble it before the camera (off-screen, of course), take a snapshot, and use it as the sprite. That way you can always have a set of sprites for each critter at hand, without the need to continuously render stuff.