Richard,
I'm not going to tell you that it's workable or not,
I have no honest way of doing so. At least not without
seeing your chimney, or rather putting my hands on it to
see what condition it's in and how well it's put together,
etc, etc. The most realistic thing anyone can tell you is
that in general, chimneys are not built to support anything
except themselves, and do not do well if large 'wind loads'
are placed on them.
The other side of that coin is that people have 'gotten-
away' with mounting antennas on chimneys for years with no
problems at all. Just depends on the chimney and how much
'stuff' you try to tack onto them.
Something else you may not have thought of is what does the
smoke and other combustion products do to the antennas and
rotor on the chimney? Of course, that depends on if you ever
use the 'fireplace', or not. What you burn if you do use it,
and how often you have it cleaned (if ever). You haven't
lived until you've had a chimney fire (LOL), inside or outside
the flue!
'Doc