It is possible for the FX2LP to behave as the SX2. In this case the SX2 handles the enumeration and then basically just acts as a data transfer medium.
The same thing can be done with the FX2. The code to handle the enumeration and send the descriptor file is already written for the developer. So, one can directly make changes in the descriptor file (as opposed to doing it with an external master as the FX2 has an internal core)
Once the FX2 enumerates, one will merely have to configure it in the slave FIFO AUTO mode. Doing this essentially gets the 8051 core out of the way, and the data path will essentially be the same as it was in the case of the SX2.