MEP is left to blaze its own trail in a whole lot of regards in Revit.
I can say that I think a good tip to start off with is:
You want a Duct Accessory family type.
You'll want to either try a face based family template from kickoff, or you can start off totally generic, set it to "Attaches To", and use a duct connector at the attachment point.
When I have more time, I can post more detailed info, but hopefully that's of a little help.