One of my tanks is supplied by a *very* awkwardly placed outlet strip. Same problem, if I cut power at the main outlet I lose the lights I need to work on the tank. Solution for me was to use a piece of extra hose to create a closed system for the filter--slip one end over the outlet, slip the other over the intake and the filter is left to chug happily away on it's own while I muck about with the tank itself.
Bonus step: before closing the loop, I use the hose attached to the outlet as a sort of leaf blower--stirring debris up off the substrate and out from around the base of plants/under driftwood, etc making it easier to remove during the water change.
For my nano tank with its HOB filter, I just slip a plastic baggie under the intake/outlet, lift the hob enough to put the edge of the bag under it, and let the weight of the filter hold it in place while I work. I do the same thing while pruning to keep the flow from blowing the trimmings around--especially all those tiny HC leaves!