But seriously, the tiles are beautiful. I agree that the last one is not as rich as the others, and clearly didn't get reduced as well.
I also agree that adding the combustible to the very hot kiln platform could be quite dangerous, even somewhat explosive like you mentioned. Of course, this would all depend on the type of combustible you use. Materials with fine particles like saw dust and such will ignite very quickly and dangerously. But what if you put in a thick stick or small log? It would smolder and smoke quickly, but would not explode, and wouldn't make a big mess. The charred remains could probably be used several times. A thick roll of newspaper might work, too. Anything that doesn't combust too quickly. You could possibly even just put the kiln top back on to contain the reduction, although it may cause everything to cool too slowly. But a large metal trash can could work, too.
I might have to build a second Raku kiln (along the lines of the trash can model OldLady describes above) just to experiment with this. I don't think that easy-riser contraption I use would allow for safe insertion of a trash can (or any other large vessel)... but I really like the idea of reducing on-site.