I've never heard of putting sumps on backup batteries because they typically are larger (in my experience) and I see little efficiency in doing so. Obviously, deep cycle marine batteries are going to be far cheaper than a standard set of backups as used with UPS solutions. The problem is, most Sumps are 120v AC and you would lose a lot in the inverter processes.
One solution would be to size your pump up...see how much water that pump is rated for in gallons per minute and get 2 or 3 bilge pumps to go with your marine battery. Bilge pumps are pretty efficient on battery life, are small, and can be purchased for very little:
http://www.walmart.com/ip/Shoreline-...ingMethod=p13n
Since the application is backup and temporary until power is restored, you can invest a little and add additional batteries/pumps if you see a need for more pumping power or additional pumps in other locations.