Alternative theory #1 (Speculative): It could be one of the Visored that didn't go to Soul Society. Remember, Grimmjow isn't really so fond of using his sword, and Hachi could probably get them to Hueco Mundo, even without a hand.
Alternative Theory #2 (More likely than you'd think): Gee, that blast looked a lot like a Getsuga, didn't it...
Alternative Theory #3 (The REALLY out there, and kind of likely, one): Ashido Kano. If you watched the anime, there was this weird filler arc during the Hueco Mundo fracas that featured a forest of Menos and a rogue shinigami. This guy. Now, unlike other filler characters, this guy was canon, created by Kubo, and was supposed to be used in the manga, but wasn't due to time constraints.
In addition, he was supposed to be the 10th Division captain before Hitsugaya that disappeared before Turn Back The Pendulum (This is possibly Apocryphal. I've read this on the net, but I haven't been able to find the interview yet). Kubo might just be using a character that wasn't exactly introduced in the manga, but ha always wanted to use to begin with.