Found a simple way to get an udp stream into OBS without using NDI. You just have to open the stream with VLC, then save it to a VLC playlist (.xspf). Make sure that the stream is the only track on the playlist. After saving the playlist, you can close VLC. From OBS, add a VLC video source and select the .xspf playlist as media file. You can change the Visibility behavior to "Always play even when not visible" if desired. This method also works with RTSP streams, and likely with other streams that VLC can play.