I think the cold is only a part of the problem. Washingtonia filifera is also somewhat sensitive to excessively wet soils (warm or cold). I would have suggested that you plant it almost in a raised bed situation, with a generous amount of sand worked in and mulched. If you had it adjacent to the south side of your house, even better. Also, you might try the Christmas lights approach for added warmth (that's what I'm planning to do with my Trachycarpus palms).

You would need pipe heating cables to keep a W. robusta alive through a normal N.J. winter. There will be visits to outdoor palm plantings on Sept. 30th (plant swap day).