Has anybody used TVHeadend and IPTV encoders to integrate proprietary IPTV streams link SlingTV, HuluTV, Youtube TV, etc into a single standard IPTV app? You'd need an IPTV encoder for each concurrent stream and a channel changer script. I had done something like this in a test environment using a Tivo as a source for DRM'd channels that my HDHomerun Prime couldn't pick up so I know it's feasible but maybe not so practical.
I manage a commercial IPTV product, and we have the ability to enter a web based stream as one of the channels in our app. I would suspect that you can do that with most IPTV apps.
In the case of IPTV apps that you enter an .m3u file to get your channels, you could edit the file and add whatever streams you want.
There is a project that has collected several web streams into a single .m3u file They are all free and legal and information can be found here. https://thefrugalstreamer.blogspot.com/ click on the link to F2V TV on the right side to get the stream and more information about what is there.