The Multiplayer NARC Patch tools in NSMBe seem to work for the levels, Tileset 1 and the backgrounds only. When I try to use e.g. a Mini-Pipe, it will become a white rectangle in-game.

I think Mvsl loads only I_dokan.nsbtx, or rather the pipe's graphics.

For example, as I recall, if you use the Desert Tileset, and use the quicksand, in-game it will shows a white rectangle, but the quicksand will work. The same should happen for the end-of-level "castle".

This probably doesn't help, but have you tried the no sprite sets MvsL hack?

Yep, I did

Sprite Sets aren't related to Jyotyu at all, so aplying that ASM hack is compleatly useless.

Multiplayer does not load all .nsbtx files which are hardcoded ontop the map 16 if I remember correctly. The only way to get those pipe parts in Multiplayer is either just placing them into a regular tileset or ASM.
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GitHub - Kuribo64 - YouTube

I tried change the tileset and it works.

But I couldn't make it functional. Unless with some ASM, a little bit out of my newbie skills, anyway.

Out of topic, but related in some way. When you will open to the community a download link of this wonderful branch?

He hasn't finished it, and he won't either. If he decides to make the code open-source, someone else could finish it.