how to use a special method to lookup a field just in the web service?

In Launchpad, BugAttachment.message needs to be an IIndexedMessage, which cannot be served by the regular model class (the DB returns IMessage); it needs to be IIndexedMessage to generate the canonical url efficiently (https://bugs.edge.launchpad.net/malone/+bug/618849).

is there any way to say 'in the webservice, when looking up the message attribute, call function XYZ instead'? That would let the regular message attribute stay as it was rather than being called _message and having a property to access it. (I'll attach the MP showing how I did this in a minute).