You will want to test them a little to see 1) If the brush will even work with NiMH cells. Some devices have problems with the lower voltage 2) How long the brushes will work on one charge. Can you charge and go, or are you going to have to bring a charger along?
Just as an example of #1: I have some keychain lights that use a single AAA cell. In the interests of economy, I tried one with an NiMH cell. It lights up, but only at the low setting. There apparently isn't enough in the cell to enable the high setting to work. It lights up, but isn't any brighter than the low setting.