Well, the problem is that RTCD is not part of the regular events or alerting system, so its not very customizable. Nor could I find an alerting condition that was "Config Changed", although that might be a really nice addition!! Maybe you should create a feature request with this...
Now, that being said, there is quite often a way to do it even if they don't leave you a clear path to it. The custom property is a good idea. I would turn off notifications by RTCD and have a custom property that signifies whether or not you want to send an email for a config change. Then, write a custom SWQL (or SQL) query to do the detection of a config. That being said, you might get some duplicate emails, but with some adjustments probably not.
The query I'd use is something like this, which looks for something in the TransferResults table that was done by RealtimeNotification where the config was downloaded in the last 5 minutes (ie: 300 seconds). Include whatever relevant fields you want in your email and such and filter on your custom field, which I'd do as part of the query below, and you should have what you're looking for...
SELECT TR.RequestedConfigType, TR.ConfigID, TR.ConfigArchive.DownloadTime, N.Caption, N.IPAddress
FROM NCM.TransferResults TR JOIN Orion.Nodes N ON (TR.NodeProperties.CoreNodeID = N.NodeID)
WHERE (UserName = "RealtimeNotification') AND (SecondDiff(TR.ConfigArchive.DownloadTime, GetDate()) < 300)
I'd think it should work pretty well. One caveat is if you go into "Transfer Results" in the web GUI And clear out the results, it might prevent an email from being generated depending on the timing. That can either be a plus or a minus depending on your perspective!!
Good luck!!