...some loose thoughts, I really do not know enough about it but the word apple events comes to mind ... might be an option to at least discover that something is happening in a watched folder ... then check if a file has disappeared
a second idea is to "simply" link hazel to cron, or add a timed event option to the rules (might be useful for other purposes), sort of, every so often check for missing files with a database.. (a plug-in module for hazel ? : -) for me, a simple solution is "find PATH to Folder > dir.txt" this produce a list with paths to everything in that folder (with 40.000 files this is about 200 kb) one can then do this at intervals and use "diff" to compare if anything has changed since last "find". Not a hazel solution, but the concept might work.
in my specific case, hazel do not need to do any action on the file that was deleted, but I need to know which file it was with path. I then, with a script, replace head of path and with sed change extension from tif to jpg, and then delete the jpg file, this is essentially a synchronize function but as it consist of different kind of files I can not use a normal sync program..
if a file is moved hazel simply discovers this as a new file (and then with an applescript copies the file across) (in other words, I do not need to track the file)
a bit messy to explain all this here, I will try to write a manual for this archive soon, will mail it to you once done. For me, hazel in combination with zenphoto and some scripting has produced roughly the same functionality as a professional archiving program, (which would have cost about 10.000 dollar to set up ... : -)