I think that I have it:

http://www.sims2wiki.info/wiki.php?title=FPL

0xAB4BA572 FPL - Fence Post Layer

Data Section:
DWORD Count N
repeat N times
DWORD FloorLevel (0 = ground, 1 = next floor up, etc)
FLOAT X
FLOAT Y
DWORD FenceType

This is a pretty simple structure, so it should be easy to fix. Most likely, the first float is X, the second is Y, and the two DWORDs are irrelevant to the LE.

[Update:]

Structure definition above was updated based on research.

If you want to remove all fence posts on the lot, just delete the FPL record using SimPE. If you subsequently build new fences, a new FPL record will be generated.