So that's just what I did.
Sorry if it's hard to see from the picture, but the letters on the left are from a .tlx file I created called "Monsters: Normal Size" It contains markers for every letter in the alphabet, and the marker is one 1" square large. To use them, I created a new layer on top of my map, and set transparency to "100%". It's on this new layer that I added the monster tokens.
This has its pros and cons.
One nice thing about doing this is that the monster tokens are always on top of the tiles. If I didn't do this, then any tile I add AFTER adding a monster token will be ON TOP of the monster token.
One negative aspect to this is that I have to set transparency of ALL layers to "100%", making it harder to do the actual planning. It also means that I wouldn't be able to see the tile layer above or below the layer I'm editing.
I think for now the negative is not a big deal. I can temporarily adjust transparencies to create my images, and I can slide layers up or down if I really need to see them. In any case, it proves that I can kind of be done.
I'm going to take a stab at modifying the markers to include monster tokens. This might be more what I'm looking for. It doesn't look like I can just add new .wwz files like I can .tlx files though.
EDIT: It's great to know I can make .tlx files, but it turns out I can easily add to an existing .wwz file. Adding the markers to generic_markers.wwz was trivial, and works out much nicer - the tokens will always be ON TOP of the tiles, and it will display the whole set of markers in a much more compact fashion. No more fiddling with transparencies, and no more fiddling with extra layers!