Docs for DEFINE tag, under System Defined Variables: "All variables listed below are global in nature, unless explicitly stated within the variable specific documentation, and can be used in any formula, in any file, in any gamemode."
BASESPELLSTAT line: This is the mod for the class's base spell stat.
No special limitation mentioned. Tom confirmed that this variable is only valid in the CLASS - this should be specifically noted here.
... not a genuine causative-type cause, but a deal of fussing & message board time because it didn't seem to be clear anywhere that BASESPELLSTAT was class-file only (perhaps it's obvious to higher mortals; I guess it does makes sense when you think about it!).
Fast work - congrats!