On the other hand they could have used 96bits of block pointer and 32bits of meta data in a sort of tagged reference or capability system, instead of shuffling around a bunch of high order zero bytes forever.
Assuming the tagged reference or capability system was built, wouldn't it need software to take advantage of it? If it's not actively used, no real point having it over more block pointer space - and I doubt significant amounts of software would use such a filesystem-specific feature.
Most CPUs do not support 128-bit integer math. They would do do 64-bit integer ops with carry. In most architectures that would be no different in code size from a 64-bit op followed by a 32-bit op.
Very complex compilers and/or cisc decoders on superscaler processors could theoretically rewrite some 128-bit to 32-bit and run the computations concurrently with other 128-bit computations.