Hello @simon17 welcome to the community!
Unfortunately, as far I can tell this is not possible - Even if you temporarily disable organization for a specific application, and use the ROP flow + a rule to manually add a custom claim of org_id, you won’t be able to as org_id is subject to restrictions.
The only option I can think of while using the ROP flow is to add a custom claim with an org_id but naming it something else, like org_id1 - Obviously not ideal, but an option if you can work around this in testing.
function (user, context, callback) {
context.accessToken.org_id1 = "your_org_id";
context.idToken.org_id1 = "your_org_id";
return callback(null, user, context);
}