I want to help Outlook 2010 thread my emails. My understanding is that it bases the conversation view off of the conversationTopic property of the MailIte
Was looking for pretty much the exact same thing, this doesn't seem to be possible with normally exposed objects as you point out, but VBA macro + Outlook Redemption allows conversation topic to be tweaked easily. Bonus, the original message subject is unchanged, but the messages still show in a nice neat conversation group.
Something like this, thrown into VBA Macro and then run this script as a Rule action when messages are received with whatever criteria you determine:
Sub MsgProcess(msg As MailItem)
Dim oNS As Object
Dim oRDOSess As Object
Dim oRDOItem As Object
Dim sEntryID As String
Dim sStoreID As String
Dim NewConversationTopic As String
Set oRDOSess = CreateObject("Redemption.RDOSession")
Set oNS = Nothing
Set oNS = Outlook.GetNamespace("MAPI")
oNS.Logon
oRDOSess.MAPIOBJECT = oNS.MAPIOBJECT
sEntryID = msg.EntryID
sStoreID = msg.Parent.StoreID
Set oRDOItem = oRDOSess.GetMessageFromID(sEntryID, sStoreID)
'Apply what modifications to topic you want here - dumb example string manipulation shown
NewConversationTopic = Replace(oRDOItem.ConversationTopic, "BLACK", "WHITE")
oRDOItem.ConversationTopic = NewConversationTopic
oRDOItem.Save
End Sub