I were thinking about framing before looking at your last changes to PEP 3154 and I have two alternative propositions.

1. Pack picked items in blocks of some predefined (or specified at the start with the BLOCKSIZE opcode) size. Only some large data (long strings, large integers) can cross the boundary between blocks. In all other cases the block should be padded with the NOP opcode.

2. A similar to your proposition, but frames should be declared with a special PREFETCH opcode (with 2- or 4-bytes argument). Large data pickled outside frames (this prevents doublecopying). Opcode and size of large data object can (should?) be included in the previous frame.