P2SH (pay to script hash) within a script?
No, P2SH can only be invoked using the exact magic pattern described in BIP16 in the scriptPubKey and not using any variant of it, nor recursively inside the P2SH redeemScript itself.

Earlier proposals (see OP_EVAL) existed that did allow much more flexibility, but it was hard to give strong guarantees about execution time and memory in such a model.