technically, the majority of mello roos is not tax deductible because the majority of it is pay for capital improvements (roads, sidewalks etc), certain parts are deductible but i forget which. with that said, california has said that they will not look for this specifically so your chances of getting audited for this item by CA are slim to none. if your tax return is simple the feds likely would not audit you to take a look at your property tax deduction, but if you have a complicated return and they audit you for something else they may take a look at it.
here is my belief though, if you cant deduct it and you are paying for capital improvements you should be able to add the costs of the MR to the value of your house and increase your tax basis to reduce any tax potential liability down the road, if i were to ever sell and be able to benefit from adding the MR to my basis i would give it a shot, seems like a reasonable argument, worst case i get audited and pay back taxes.