I'd argue an empty set of general-purpose architectural registers, a set of several base/index registers, instructions that take both their sources and destinations from indexed-memory operands, coupled with the stack engine, internal register renaming and memory remapping (to the unexposed register file with oh I don't know, 512 or more of non-architectural general-purpose registers) would work as well, if not better. You get the effect of the register windows by simply passing arguments on the stack (which is actually backed by invisible registers), and its easier to resolve dependencies when you use essentially unbounded number of stack slots instead of the fixed number of registers.
Look at 6502: making an implementation for it with the zero page backed by register file instead of RAM is entirely possible.
Reordering RISC-like instructions is also lot easier than reordering complex ones. A larger set of architectural registers also makes that easier.