It seems to me that plastination is unlikely to preserve chemical details of synaptic protein expression. We really don't know how crucial that is to personal identity, but I suspect it might be possible to have a two stage process, in which the brain is first flooded with dyes, which diffuse into the neural tissue, and bond onto specific synaptic proteins. Then, when the brain is preserved in resin, you have gross visible records of neural and synaptic chemical details.

On a related note, might it be possible to safely break the brain down into more manageable fragments for preservation, by cutting through the sections of the brain that are architecturally simplest, or vary the least between individuals? The corpus callosum and lower mid brain seems like a good place to start.