I'm trying to setup something like the following:

This requires a config option to set The Erebus portal's destination dimension. So I should be able to create a portal in The Erebus, and it leads into the nether, and vice-versa, and the portal shouldn't work in any other dimension. The Betweenlands has such a config option. (It also has an option for portals working from more than one other dimension; not sure if the tree remembers its source or not.)

Want to make a PR? If not, no worries. ;)
I'm willing to add this ofc, but it may take a while to do - IRL, other mods and mod-pack stuff etc. are eating in to my spare time. (Writing this at 2.30am and need to zzzz, will take a look after IRL work one evening this week). Erebus is lacking in some much needed attention :P