I know at certain %'s, cargo -> uthrow -> giant punch is legit, and I've seen a few DK's land cargo -> jump -> bthrow -> fair (who knows if that is legit, I've seen it work/gotten it to work a few too many times). It seems like your best bet would be to go for one or two uairs depending on there %, then landing and waiting to see how they react. Marth's don't like it when you are below them and he is in the air trying to land, so bait his reaction and punish him further. Maybe let him land and go for a regrab, or get underneath him and go for a upB.
I don't think you're going to see DK getting those uair juggles like vs. ffers... so don't expect too much.