Is it possible to change the activation dimension blacklist in group_data.json to a whitelist?

Okay, from further work it seems the only dimension that actually works is dim0, no matter the config?

The best solution is to enable blacklistAllUndefinedDimensions in group_data.json.

What about the latter issue though, where the portal doesn't activate outside of dim0 even with the blacklist empty?

Have you defined portals for each dimension?

I have 0..json in one of the groups, defining a glass portal leading to dimension 8. There are no blacklisted dimensions for activation. It can only be activated in the overworld. Any idea why?

You'll have to show me your 0.json and group_data.json.

0.json
group_data.json

Github doesn't directly support json files, so I changed the extension. Same contents though.

Your first file is a copy of the second.

0.json

I'm not sure why, I've just checked contents locally and they show what they should. Is this better?

Does the vanilla Nether portal activate in other dimensions?

Does the vanilla Nether portal activate in other dimensions?

No, it doesn't.