I think it'd be better to get one of those water level detector/alarm things. That way you'd keep the filtration going.
I keep my always leaky filter in a 10g tank. It would displace a lot of that 10g so it's still not going to hold that much, but it's also easy to see if something starts accumulating. (It tends to leak out the gasket but only if it's not 100% level. As it is now, the 10g is dry.) An alarm could let you know it's getting too high and you could deal with it without running the risk that the power stays off for a long time before you notice and end up with dead stinky junk in the filter.
You could also use a shower liner and make a nice large catch-all.
