the Microsoft Macro Assembler 32-bit (from the Visual Studio 2010 x86 build tools) has updated its preprocessor to use "extern/extrn functionhere:proc" already and the current maintainer of the community created MASM SDK is still using the old assembler AND that doesn't include some of its modern functions/features (one of which was the feature that I mentioned)
if they continue to build up on that old assembler version (their macros are too stacked), maybe a new feature included that addresses an update can break some of the important stuffs in their SDK.
just saying.