deep sand bed

You mean hyper specific gravity like the Red Sea high? If so, I would surmise the answer is yes the deep sand bed will still function. Some strains of bacteria will most likely establish themselves in the deep sand bed and the deep sand bed will still maintain aerobic and anaerobic zones, which is one of the main functions of a "deep" sand bed.
 
The main parameters determining the usefulness of a deep sand are grain size, surface area, depth and availability of food for the micro-fauna. If your QT tank is, for example, ten gallons and is only rarely occupied...I'd guess it would be too small an area and not enough nutrients to keep the fauna going. IMHO...a waste of time. Much better to keep a sponge filter in the sump of your main tank that you can easily move to your QT tank when you are hosting a new arrival.
 
I would run the QT bare (the only tank I would run bare). You don't want to set up an environment where bad things could hide and grow.
 
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