I changed the generated code to call:
format(x [, spec])

instead of:
x.__format__(spec)

The reason is that the correct way to call __format__ is actually:
type(x).__format__(x, spec)

That is, the __format__ lookup is done on the type, not the instance. From the earlier example, the disassembled code is now:

>>> dis.dis("f'a={a}'")
  1           0 LOAD_CONST               0 ('')
              3 LOAD_ATTR                0 (join)
              6 LOAD_CONST               1 ('a=')
              9 LOAD_GLOBAL              1 (format)
             12 LOAD_NAME                2 (a)
             15 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             18 BUILD_LIST               2
             21 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             24 RETURN_VALUE

The simplest way to make the lookup correctly is just to call format() itself, which does the right thing.

I still have a concept of adding opcodes to handle FormattedValue and JoinedStr nodes, but that's an optimization for later, if ever.