Jason:
We do not plan to release un updated version of FAST/AeroDyn with tower aerodynamic (drag) loading in the next several months, so, I suggest you add the feature yourself if you need it now.
Unfortunately, the user-defined tower loading option of FAST is not currently documented in the FAST User’s Guide. Some information is given in my September 12, 2011 post in the following forum topic: Simulink. When the user-defined tower loading option is enabled, FAST will call routine UserTwrLd separately for each tower node for each structural time step. Hopefully the sample source code of UserTwrLd that is supplied with the FAST archive is self-explanatory. Note that the routine specifically says that it is not meant for tower aerodynamics, but this is only because we plan to build tower aerodynamics into AeroDyn at some point. Until then, the routine can be used for this purpose.
From within UserTwrLd, you can call AeroDyn’s WindInf_GetVelocity function to get the wind speed at any given point in space at any given time. You can use this e.g. to access the data from hub-height wind files or turbulent wind files including at points along the tower. You can, of course, use this wind speed to calculate the wind drag force that will be applied to the tower from within UserTwrLd.
We’d be pleased if you send us your code, which may help us expedite our own code-development effort.