Is there any reason that this combo wouldnt work in a Tully deck with DP used as an OOH trigger? Does the challenge phase need to end naturally in order to trigger Riverrun?

The combo works. Drowned Prophet effectively jumps to the "end phase" framework window, which is where Riverrun is resolved.

Hugely expensive combo at 5-gold and 8-influence for Drowned Prophet OOH, though.