Actually Hazel already (half way) suports OpenMeta.
You can take use of "Tags" (OpenMeta) in "Conditions - Other" and set a rule according to it, if a file contains a certain tag or if a tag is missing (blank).
Additionally, as Mr_Noodle post it in this thread, you can use OpenMeta's "omtool" to add tag to a file.
Also, it is easy to create, let say, a small OMC (OMCEdit) standalone app which could, under certain rules, pop up a dialog where you can set a tag for the file just added to watched folder...
Edo
P.S.
Here it is - "OMC_Tagger":
http://drop.io/OMC_Tagger/asset/omc-tagger-app-zipBefore running it, make sure that "omtool" is placed in your "Applications" folder. Then, set a rule "Open file" with OMC_Tagger.
Also it is recommended to check "Quit after executing the command" in OMC_Tagger "Preferences".
Note: It can tag only one file at a time if used in Hazelrules, otherwise it can handle multiple files if used directly from OMC_Tagger.