Thanks for your help, James! I did more research and have found a little more information. I'll attach it here for anyone else interested in digging through and reading it for themselves.
The IFC does not address this issue directly, rather it points to the NFPA to give guidance. From what I have found so far, the NFPA only requires grain to be separated if the grain storage area is greater than 5000 sq ft.
IFC Chp 22 - Combustible Dust Producing Procedures
http://publicecodes.cyberregs.com/icod/ifc/2012/icod_ifc_2012_22_par006.htm
2204.1 Standards. The fire code official is authorized to enforce applicable provisions of the codes and standards listed in Table 2204.1 to prevent and control dust explosions. TABLE 2204.1 EXPLOSION PROTECTION STANDARDS
STANDARD SUBJECT
NFPA 61 Agricultural and Food Products
NFPA 69 Explosion Prevention
NFPA 70 National Electrical Code
NFPA 85 Boiler and Combustion System Hazards
NFPA 120 Coal Preparation Plants
NFPA 484 Combustible Metals, Metal Powders and Metal Dusts
NFPA 654 Manufacturing, Processing and Handling of Combustible Particulate Solids
NFPA 655 Prevention of Sulfur Fires and Explosions
NFPA 664 Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities
NFPA - 61
http://iepi.neu.edu.cn/Download/Standards/NFPA/NFPA%2061-2008%20Standard%20for%20the%20Prevention%20of%20Fires%20and%20Dust%20Explosions%20in%20Agricultural%20and%20Food%20Processing%20Facilities.pdf
4.3.1 - Interior Wall Construction
Storage areas larger than 5000 sq ft and containing packaging , bagging, palletizing, and pelleting equipment shall be cut off from all other areas with fire barrier walls designed for a minimum fire resistance of 2 hours and designed in accordance with NFPA 5000, Building Construction and Safety Code Chapter 8, Fire-Resistive Materials and Construction.
NFPA 654
http://www.ishn.com/articles/98439-understanding-the-changes-to-dust-explosion-standard-nfpa-654
This references testing the risks associated with combustible dust. I think this will come down to being able to convince your local authorities that the dust produced by your hammermill does not present a hazard.