I think my lack of knowledge on the subject is creating confusion.

My requirement is to detect emals that have a set of keywords. When such
messages are detected by the service I wrote that runs on Exchange Server, it
adds a record into the database (that's also stored on Exchange server
machine). This flagging process takes place immediately when a message is
received by Exchange Server.

Then I want to avoid users from seeing such flagged message contents, by
runnig a check at the time of loading the message. Every message selection
event should triger a database check against the message's ID and if it's a
marked message, I want to prevent the user from seeing the contents and show
a generic message such as "This message is under auditor's review and will be
available later.". The same thing is done by OWA customization so that OWA
users wont see the contents either.

With the amount of knowledge I had on the subject, I assumed by creating a
new IPM.Notes form which runs the necessary checking and replacement
operations as a set of macros, and then publishing these forms into
Organizational Forms Library I'll be able to achieve my requirement. However
from the information I got from you I'm in doubt whether my approach is
correct. If I can avoid installing anything on users' machines and still do
what I want, it'll be great.