Can this "roll roofing" be installed over existing cedar decking to act as a new surface?

I do not think the rolled roofing is what you want to use. It is not designed to be walked on on a regular basis. You might look into the rolls of self-adhesive outdoor stair tread cover.

Required roof decks for shingle application are 6” maximum width, 25/32” minimum thickness wood sheathing, 3/8” minimum thickness Exposure 1 grade plywood sheathing or 7/16” minimum thickness non-veneer structured panels (oriented strand board panels or waferboard panels). Sheathing must be spaced a minimum 1/8” and a maximum 1/4”. Use plywood decking and non-veneer structural panels recommended by the American Plywood Association, Underwriters Laboratories, Inc., or local building codes. Owens Corning stands behind the manufactured quality of it’s shingles regardless of the roof deck type. Should there be manufacturing defects in Owens Corning shingles that would reduce their normal life span, we will stand behind them to the limits of our warranty. However, Owens Corning will accept no responsibility for any problems resulting from failure of the roof decking material not directly attributable to a manufacturing defect in the shingles themselves. Owens Corning shingles have been tested and rated as Class A by Underwriters Laboratories when applied over recommended decks. If other decks are used, the resulting construction may not qualify as Class A. Regardless of the deck type used, the roofing installer must: 1. Install the deck material in strict compliance with the deck manufacturer’s instructions. 2. Prevent the deck from getting wet before, during, and after installation. 3. Ensure ventilation meets FHA Minimum Property Standards.