There are a couple of simple ways that this could be done. One would be to simply use a vocoder in conjunction with the DX100. The other would be to just run the DX100 or any sound for that matter into a conventional guitarist's sealed speaker talk box (a la Peter Frampton, Jeff Beck etc). FM through one of those would sound very cool, though those things will vibrate your filling out over time .... really !!!

But credos to Mike for working on programing it, as those sort of sounds are certainly possible using FM. One of the early Yamaha promotional demonstrations featured a DX7 counting to 10. In fact the recent FS1R from Yamaha with advanced FM and formant shaping is supposed to be excellent at vocal and vowel type sounds.