I've been testing the IQ-XM here at the shop and talking with people at Busch & Müller. It turns out that the battery can be charged while it's powering the headlight. What's required is a power bank with a dedicated output. Here's the word from B&M.
"You can charge it with a secondary power bank that uses a separate (dedicated) usb port to charge a 3rd party device. If it uses the same port to charge and be charged, then there is no guarantee that energy will be transferred in the required direction, because the speed Battery does that also."