Interesting question and one where I believe the tax law is not clear. However, I'll give you the answer, which I believe reflects the intent of the law. Also, this discussion is related solely to rental income deductions, since maintenance fees would not otherwise be deductible.
First, to differentiate, if this had been a special assessment of property tax for a local area, it would not be deductible. The law is clear on that, for the reason that such assessments are almost always for improvements such as sewers, road improvements or other items that should be capitalized and (if appropriate) depreciated for tax purposes.
To determine the deductibility of a special assessment related to timeshare maintenance fees, it's appropriate to look to the reason for the assessment. Thus, look at the letter from the HOA or management company or ask some questions.
If the assessment is solely related to current operations (unexpected repairs, etc.), the entire amount should be deductible as a current rental expense. If the entire amount is solely related to improvements, such as a new swimming pool, an expansion of the activity center or a new parking garage, the entire amount should be capitalized and depreciated as discussed above.
If, however, the letter indicates that the assessment is for both improvements and current operations, you'll need to make an allocation. The IRS would probably accept any reasonable allocation that you make.