Hello CK – Schalk,
I know that for our program at Tower many affiliates use offline materials and are very successful, in fact my best 2,3 affiliates are strictly offline and they utilize mailers and newsletters with promo codes that have their tracking info attached. Those affiliates seem very comfortable doing that and typically experience very little complications compared to site driven affiliates.
Having the Affiliate company design those materials is also a time and money saver just be sure to test registrations before doing a mass mailer to make sure the tracker codes work right.
Hope that helps