Stars only resulting in different treatment from the faction or different comparative authority across factions is a completely reasonable standard, and I strongly agree that this is a good way of doing things, it means the oldest and most-established characters won't monopolize the spotlight either. As long as it's well-articulated and consistent, that's totally fine.

I suppose what it boils down to is this: Whatever the OOC route to opportunities for participation in the plot, it needs to be clearly outlined, because as long as the process isn't visible it will feel arbitrary.

If it's not stars, that's totally fine. If it's stars on specific occasions and not on others, that's also fine. Hell, if it's just stars, I don't love that but it's at least impartial.

But it should be transparent, communicated when it's relevant, and consistently applied, regardless of what the answer is.