const options = {
    Model,
    id: ['entityId', 'otherEntityId'],
    paginate: app.get('paginate')
  }

Throws a type error, "string[] is not assignable to string", but according to the feathers-objection docs, this is how you use composite primary keys.

Unfortunately, I think fixing this also requires a change in feathers. It looks like the upstream feathers AdapterService class doesn't allow a way to override its options object type. So, even if we did

export interface ObjectionServiceOptions extends Omit<ServiceOptions, 'id'> {
  model: typeof Model;
  id: string | string[];  // <--change
  // ...
}

We would still get a type error here:

export class Service<T = any> extends AdapterService<T> implements InternalServiceMethods<T> {
  Model: typeof Model;
  options: ObjectionServiceOptions;
  // ...
}

The actual fix is probably to make the feathers AdapterService accept an additional type parameter: AdapterService<T = any, S = ServiceOptions>, or to change ServiceOptions, but I think that might be more breaking.