Well, I do have a little technical know-how, although not specific to the DLNA spec. You seem to be reading something that wasn't intended in my comments. I wasn't trying to be condescending, or otherwise talking down. If you took it that way, I apologize.
I do believe that what you list would require a wholesale change in the basic coding of the program. The MediaBrowser function is completely separate from the server function. The only network transport mechanism contained in Serviio is the DLNA standard. As I mentioned, that standard will not route across the public Internet. This is by international design. Now, could another transport module be added to allow a TCP routable packet to function outside the private network? Possible, but without knowing the inner workings of the program I couldn't guess how much work would be involved. It would also require a more robust security piece, which would be more work.
Serviio itself is not visible outside your internal network. All you can reach is a web page, which uses standard Internet protocols to display the video. No different from any other web site in that respect. The MediaBrowser is on top of Serviio, and provides a nice way to view your media from outside. But that's all you are reaching, a web site, not Serviio itself.
I'm not saying your idea doesn't have merit, but put it in this context. Serviio is a one man operation, and not his full time occupation. He has to balance the desires of his users with what can be done within a given timeframe. As you noted, based on the lack of comments it doesn't seem that there would be a lot of call for this.
Just because I personally wouldn't be likely to use something like this, doesn't mean that there isn't enough potential customers. For me, only one outbound stream would choke my Internet connection. Most Internet connections are asynchronous, meaning upload and download speeds aren't equal. While I have 50 Mbps down, I only have 5 Mbps up. Not even enough to support a single HD stream.
In closing, I wish you the best in your search. Perhaps our discussion has spurred Zip into thinking of a way that it could be done. Sometimes all it takes is someone bringing an idea forward to make the developer think about it more. That has happened on several programs I've written over the years!

Dan
LG NANO85 4K TV, Samsung JU7100 4K TV, Sony BDP-S3500, Sharp 4K Roku TV, Insignia Roku TV, Roku Ultra, Premiere and Stick, Nvidia Shield, Yamaha RX-V583 AVR.
Primary server: AMD Ryzen 5 5600GT, 32 gig ram, Windows 11 Pro, 22 TB hard drive space | Test server: Intel i5-6400, 16 gig ram, Windows 10 Pro
HOWTO: Enable debug logging HOWTO: Identify media file contents