I'll mention it to the products team, but don't think it's very likely for a couple of reasons:
First, NiMH cells are tricky to charge. There's no clean line between "okay, I'm fully charged" and "okay, I'm outgassing hydrogen and about ready to explode". A good charger uses a combination of voltage, current, and temperature to decide when to quit.
Second, we prefer to keep our power devices as interchangeable as possible because those are the pieces that fail in exciting ways. All the LiPos we carry work with all of our chargers, PowerBoosts, and Feathers. Aside from plugging in a third-party LiPo whose JST connector is wired backwards to ours, not much can go wrong. If we add multiple voltages and battery chemistries, the combinatorial issues explode. We want to remain beginner-friendly, and that's not a good place to remove the safety padding.
For people who are ready to work with power sources outside the 3.3v and 5v and generally less than 1A range, we like to refer people to
http://pololu.com/ They focus on robotics, and carry the range of devices appropriate to the next few steps up the ladder. They also lean toward an audience with more electronics experience, so they have more freedom to say "we assume you know what you're doing."