I'd like to have multipart support because exposed cables in buildings are ugly.
I'd also like an EnderIO conduit for the network, besides multipart, for modularity

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This doesnt help you at all and im sorry, but first i have to fix lots of bugs and then port to neoforge 1.21.x , and in that environment we can see what multi part mods exist

Forge has never had an official multi-part api. There was an addon in 1.11/1.12 for a hot minute maybe? that mus be what enderIO used..

If Forge 1.16+ makes one or a new supported API comes out its worth looking into then i guess

There's a CBMutlipart, but it wasn't updated for 1.5 years.
You can look at source code for EnderIO here