If your high-level objective is to fetch the new document number/version/whatever after a document has been saved into iManage then you were already on the right track
Basically you should capture the PostOnOK event of the IManFileSaveCmd itself. That will expose the actual dialog from which you can fetch the "NewDocument" property. NewDocument is merely the new instance of IManDocument (or NRTDocument) which iManage created for you, along with all the new metadata (i.e. the new doc number, version, etc)
So you need to
1. capture OnIManFileSave (I'm assuming you're monitoring the iManageExtensibility events)
2. when that event is fired start monitoring the PostOnOK event of the IManFileSaveCmd that is passed in as a parameter
3. when the PostOnOK event fires cast the pMyInterface parameter to IManFileSaveDlg
4. fetch the NewDocument property from the dialog instance
5. update your document with the new values found in NewDocument