Stuart's suggestion of sending a DSN to inform them of the error seems to
avoid the risks associated with rejections, but still accomplish your goal
of notifying people of errors.
To whom would this DSN go? The originator of the message at which time
we encountered the faulty SPF record? Most likely, that person has no
responsibility for (or even knowledge of) the SPF record, and the DSN
would probably confuse them.
There is also a downside to this. In the event that the SPF record is
broken and the particular envelope recipient is invalid (its MX record
points to a never-up mail host), you could end up with a large number of
DSN messages in your queue for bogus recipients. SPF attempts to prevent
this, and this kind of action could negate that benefit.
--Marc