I understand you want to do the peroxidase blocking and Triton pre-treatment in a uncut (half) brain.
I would say it wont work and you will end up with tissue damage. Penetration of the solutions is slower in a chunk of tissue and you will need more time to obtain the same results. More time in h2o2 will result in tissue digestion.
I'm also concerned about freezing artifacts afterwards, to mention just two big issues in your proposal.
I agree with David that solution penetration in a whole block of tissue is slower than in slice. To maximize tissue life, and to prevent freezing related damage, I suggest saturating the saggital chunk in sucrose prior to freezing, and then following thin sectioning (~40um) to perform the peroxidase and triton treatments before IHC.