That is a bit paranoid, but it could happen. I have not seen anybody do
anything that intelligent in the past couple of years. Not to say that there
arent people out there that couldn't do that but I think many have thought of
using one exploit to expose another, DDoS is the closest I have seen on any of
my honeypots. I have learned many things about what most people will try to
get into a box from the honeypots, but that is a good point. Filtering or
patching should take place on the edge and on the most critical spots on your
network.
Good Luck
Not paranoid enough. 
Not only *could* it happen, it almost certainly *is* happening.
Remember that in general, only the ankle-biter black hats get caught, just like
the police catch mostly the stupid crooks.
My co-worker Randy Marchany has been doing talks for *years* saying why
firewalls by themselves don't work - he'll ask the audience how many run firewalls,
and a lot will raise their hands... then he'll ask if they pass port 25 and/or 80, and a
lot of hands remain raised.. then he'll ask if *anybody* behind the firewall is running
an unpatched Outlook or IE... and a lot of hands remain raised, with very worried looks
as the implications sink in....