I apologize if this has already been questioned, since I have not read all of the posts:
Are there any work crews who plow sand on the beaches in the area?
I think you're referring to beach replenishment as part of ongoing coastal erosion management programs. Yes, Long Island is monitored for erosion and parts are regularly replenished with new sand. There are many, many different agencies involved at all level of government. Despite the characterization of this area as "desolate", it is on no way "abandoned".
I would expect this park's shoreline and every part of the preserved lands to be regularly inspected, and it's one of the things that's really bothering me that bodies may have been there for years undiscovered during any environmental inspections, grant applications and so on.
I do not know if burlap is used as part of a replenishment program - which is typically just dumping more sand and dredge spoils to build up the shoreline resistance, but it's a good thought.
What I do know is that no work can go ahead without a DEC permit, and the manner and materials in which replenishment has taken place (if any) would be specified in great detail in the permits from County, State, DEC, Army Corps of Engineers etc. Those permits may or may not be available online, but I would think not easily accessed without a FOIL. Given how common burlap is, the gain wouldn't be worth investment of time.
A Report called, "National Assessment of Shoreline Change: Historical
Shoreline Change along the New England and
Mid-Atlantic Coasts" was recently published on Feb 11th and includes the South Shore of Long Island at page 43. I don't know how data was collected to prepare this report, but would be very surprised if it didn't include some form of visual inspection, by air and from the ground which should have uncovered these bodies if they were there since 2007 until recently.
http://pubs.usgs.gov/of/2010/1118/pdf/of2010-1118.pdf
Just another question mark hanging over the whole sorry story.
On a different note, I woke up today thinking about whether these girls were taken for snuff movies and that's what Shannon stumbled upon. I now feel physically sick and have to step away to get it out of my head.