Hmm, is this reasonable? BOL is a very basic concept. Don't think it
should be permitted to be interfered with fields.
The idea is that a Lisp program would use fields only when the
produced effect is what the user would expect. You see this every day
in action when you type "C-x C-f" and then C-a: point stops at the
beginning of the default directory, without entering the "Find file:"
prompt. Moving point to BOL in this case would be a nuisance. (You
can still get to the beginning of line by other means, like C-b.)
That's what fields are all about. If you don't want this effect,
don't use fields; they are opt-in.