Hi Scott
As you know, I'm no expert. But I did have a Roland PMA-5 for a while. And it does exactly what you imply. That is, it reads your left hand chords from a midi keyboard and can "record" them to a backing sequence, which you can then play back and play a real time right melody to. Not like an arranger keyboard at all.
I think the QY70 might be the same. I'm not sure, but I don't think it will work the way you want it. You should consider buying one from a dealer with a good return policy. Then audtion it - if it's not what you need, back it goes.
One last thing, though not battery powered, the Korg iS40M module is very, very nice. Basically, it's the module (keyless) version of my iS40 keyboard - which I love. But I think it includes a vocal harmonizer as well. I know it's not what you're searching for, but I want to say it's a beautiful thing.
Take care...
Tom