I don't see why there should be a separation at all. It's a family day out, as soon as you start giving people the choice of whether they do the 'big rides' or not, you start putting a limit on someone's day and it creates a two-tier system.
On my first visit to Towers, I hated coasters, and I never expected I'd ride anything 'big'. In this scenario, I would have probably bought the 'small rides' pass. So, when my friends tried to convince me onto Nemesis, I would immediately have an additional cost to pay and I probably wouldn't have been on at all.
And we know that Merlin would abuse it to hell and beyond, so virtually everyone would still end up buying the high-end pass. For the amount of faff and money that could be spent implementing this, you might as well invest in decent family attractions that make those who only want to do the smaller rides not feel short-changed.
I'd imagine an annual pass wristband would likely work like Blackpool, where you have to queue up and present your annual pass at the ticket office to get a wristband and entry ticket like guests with regular tickets.
The last thing I want to see at Towers is even more reminders of money throughout the day killing the escapism.