Yes, the whole pool. Plaster isn't actually waterproof - it does a good enough job but could be much better. Just look at how many people have issues with the rebar rusting through their plaster....BaseCrete for the entire pool?? Is this an industry standard practice? My initial thought was/should be for waterline and wet tile areas. Plaster would act as a water sealant for the rest of the pool surfaces?
The other huge advantage of basecrete is it acts as a bonding agent between the shotcrete and plaster and is relatively flexible so it allows for some expansion between the shell and plaster. The two materials typically have different compressive strengths and dissimilar expansion characteristics (3-5000 psi for shotcrete and 7-10k for plaster). Add in the waterproofing characteristics and you got yourself a great product.