Ok,  I've changed the branch to:

> The ``obvious'' interfaces were:
> 
> register_to_value (frame, regnum, type, buffer)
> value_to_register (frame, regnum, type, buffer)
> 
> but that tripped up on something (now what ...?).  Dig dig.  Notice how, to preserve existing behavior, legacy_register_to_value saves the location based on what frame_register returns.  We'd have to switch to lval_reg_frame_relative.

And this time it appears to be going better.  I'll try to attribute it 
to VALUE_FRAME_ID and the use of put_frame_register (neither of which I 
did last time).

Andrew