It definitely works in my lab ..
step 1
Add IPM.Note.RSS to the Message Classes on the Directory properties in the VAC
step 2
Verify it's not ticked on the mailbox policy
step 3
Restart the archive task for good measure, and so that I can see this :
3496 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::PopulateFromExchangePolicyEntryId - Read MessageClass [Appointment] [1B688A833A0360E4FB997D0169EDABE2B1012700evault1.EV.Local] [EXC]
3497 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Entering routine
3498 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Task]
3499 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [ReturnNonReceipt]
3500 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Return Receipt]
3501 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [REPORT.IPM*]
3502 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Reply]
3503 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Phone Message]
3504 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [NonDelivery Report]
3505 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Memo]
3506 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [IPM.Task*]
3507 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [IPM.Stickynote*]
3508 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Exclude filter matched include filter I:[IPM.Note*] X:[IPM.Note.RSS]
3509 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [IPM.Contact*]
3510 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [IPM.Appointment*]
3511 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [IPM.Activity*]
3512 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Delivery Report]
3513 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Bookmark]
3514 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Rationalizing (dropping) exclude filter [Appointment]
3515 06:06:45.596 [8196] (ArchiveTask) <9700> EV:M CMessageClassPolicy::RationalizeMsgClassList - Exiting Routine [0x00000000]
You can see it's excluding IPM.Note.RSS
Now I have two test messages, one is plain old IPM.Note, the other is IPM.Note.RSS. I can see in the trace of the archiving task that the first message is processed and archived correctly. The second one is not in the trace at all (that's because of the way that the Archiving task uses MAPI restrictions to get the list of possible candidates for archiving).
Hope that helps,
Oh and by the way - an end user will still be able to able manually archive IPM.Note.RSS if you allow the store in vault button in Outlook / Outlook Web Access... unless you implement the registry key to make the clients obey the server policies.