Murray
What other filtration do you have? The water going to the DSB should be free from any detritus so it does not become a nutrient sink. From what I've read it should not be in a lit area so it does not grow any algae. what you propose would work great as a refugium, you could actually light it, grow algae include a miricle mud or dsb. etc.
The idea of a RDSB is all that is in the sand is bacteria to consume nitrate Vs the Refugium has plants and critters that fish in the display would eat if they were there. Both should reduce nitrate
Good luck which ever way you decide, you could even do both
Alan