Is there some reason we can't install some temporary pipe with a shutoff valve? The backflow should have some sort of threaded connection you could temporarily screw in some testing pipe with a shutoff. If you already have pipe installed in the outlet side of the backflow, you could either glue some test pipe in place that you cut out afterwards, or you could connect some test pipe with a compression coupler. WARNING!!! If you try something like this with a compression coupler, some other physical means is needed to hold the test pipe in place. If you don't do something to restrain the test pipe, the water pressure can cause the compression coupler to blow off.