Apologies for coming in late...
We're working on something that may help. We developed it based on feedback from sound engineers who have been using our products & it's due for release in Feb 2018. It is based on our existing MPL205 battery holsters:
http://www.inspired-energy.com/mcl205.html
& it will operate from an NH2054HD34 battery:
http://www.inspired-energy.com/nh2054.html
It will include charging circuitry built-in & it will come with an AC:DC adapter, so you could run it plugged into the wall & then if the power fails, it will seamlessly operate from the battery. (Technically the output is always from the battery anyway so there is no "switch-over" when the AC goes away).
The onboard smart charger will be fully SMBus compliant & will talk to the battery & turn off when the battery is fully charged & will reduce the charge rate if the battery is hot or cold etc...
So it will look like our long battery holsters, but the output cable will be removable via a locking connector. It will have an onboard charger & also a USB charge port for cellphones & tablets etc.
It will also have an ideal diode built-in so you can wire two or more outputs in parallel to give you hot swap capability with multiple batteries.