This sounds like it would be pretty funny.
No promises, but I'll probably work on this if I have some free time.
EDIT: I have some free time now, I'll get to work on this. I'll give you the option of specifying which usergroups see the posts in which ways.
Hmm. Well, this was an interesting first test.
Check out
this thread on my boards (yes, I use my live boards for coding and testing. Sue me). I had had the hack on for all usergroups, not just guests at the time I made that, because I was testing it in the first place. As you can see, it cached the converted posts.
I've got a good idea what I'm doing now though.