Not sure about the HOB filter issue on your QT, but my guess is a little water movement in there is probably not a bad thing if an outage is longer than a few hours. On the other hand, I've had a pail of old salt water with ball of chaeto sitting for weeks & when I finally got around to emptying it & grabbed the ball of chaeto, there were still pods wiggling around in there. This is with the water stagnant & unheated in a pail in a cool basement. Pods aren't bacteria, but I suspect there was still plenty of that alive in there as well. Who knows what really goes on at a microscopic level?
I have 2 of the automatic battery backup air pumps in my display, but instead of the flexible tubing with air stone, I use rigid air tubes with air output near the substrate at the bottom of the tank - no air stone. This way larger bubbles are produced, which creates a tad more water movement than would the fine bubbles coming out of an air stone. Of course without the air stone you need an attachment near the end of the tube to keep it from rising up once the air pump kicks in. I have a couple of magnetic clips for that purpose, suction cups don't last long term.
Another item that's not specifically meant as a power outage backup but could be used as such is an 'air lift'. You could hook one of these to the battery powered air pump as well, more for water movement & less for aeration. Here's a description I copied from the net in order to build one:
A simple device to generate water circulation where you don't want to use a powerhead because of the fragile life forms there which might be harmed by a powerhead or pump impeller.
I used six of them on an acrylic sheet to create a race track circulation in a brine shrimp tank.
I got the plans from The Plankton Culture Manual by Frank H. Hoff & Terry W. Snell. This is a very good book if you are interested in raising any type of plankton.
All that is required to make an air lift is a piece of PVC pipe with one end cut at a 45 degree angle and a PVC elbow on the other end with a 3/16" hole drilled in the top corner of it.
Insert a piece of rigid airline in this hole and push it down almost to the high side of the 45 degree opening.
Set the length of the tube so that the elbow is half out of the water.
Hook an air pump to the top of the rigid airline and adjust it so that you get a slow stream of large bubbles.
This is a nice addition to a refugia.
And one of two I built. This is a shorter one, but they can be made to whatever length suits your purpose/height of tank/fuge. You won't get a any bubbles in the water column with this since they stay in the tube until they surface, where they pretty much pop straight away.
