You are wise to be thinking ahead - the College Board can be fickle. However, from my experience, they are more concerned with the evaluative data from the psychiatrist, neuropsychologist etc than the 504 plan itself. Although it IS important to show a history of having the accommodations that you are seeking (ie extra time).
There is no federally mandated form for the 504 as there is with an IEP. Under an IEP you must list the primary disability to show eligibility. There is no such requirement for the 504 - AND with both - it is understood that a student can have more than one disability (and often they do). However, if dysgraphia is the primary issue, I agree that it makes sense to have the documentation reflect this. Remember, to qualify for dysgraphia (SLD in written expression) under IDEA you must show that they student has the disability AND needs specialized instruction/related services/other modifications to access the curriculum. Specialized instruction is not needed under a 504 - only accommodations.
These resources might help:
Americans with Disabilities Act Amendments Act of 2008
http://www.ncld.org/on-capitol-hill...th-disabilities-act-amendments-act-adaaaBest of luck!
(BTW - I had much less trouble with the College Board than the ACT for accommodations)
