Vendors and OEMs will no doubt use all the strategies you suggest, and others no one has though of. One we have considered for our own products is to use Multi-chip Module technology to put a ROM in the same package with the CPU, containing a pre-specialized in-memory specializer for all the (possibly different) cores in the package. The ROM would also contain the emulation code for operations not present in the hardware, and minimal initial code for things like trap and fault handlers and the power-up sequences, to give the BIOS a canonical platform to work on regardless of which family members are present.
The Mill approach to binary compatibility is not a panacea; we are still sensitive to the genForm distribution format, which must be stable and forward-compatible across the family, just a binary format must be stable and forward-compatible across other families such as the IBM mainframes. GenForm is not yet stable even for us; we are actively at work on it, but it will be done long before first product ships.