This is an update of my earlier posting here. I've finetuned the concept and come up with a solution of sorts, but I'm still hoping someone might have a better idea.
Basically, I'm creating a subscription site like an online training site. Here's how it will work: every new subscriber (no matter when they subscribe) will first receive lesson #1, then lesson #2 a week later and so on. Then at the end of the course (lesson #30, after 30 weeks) the subscription expires.
It's easy enough to add subscriptions for content with Ubercart, but the hard part is restricting access to content so that subscribers are forced to follow the training sequence. Whichever way I look at it, this involves creating (and then managing) a lot of roles.
Here's the best solution I can come up with so far:

• Use Content Access module to restrict each lesson to an equivalent subscriber role. For example, "lesson #1 subscriber" role gains access to lesson #1, "lesson 2 subscriber" role gains access to lesson #2 (while also retaining access to lesson #1), and so on.
• Use Ubercart to sell the "lesson #1 subscriber" role.
• Use the Administer Users by Role module to upgrade users to the next subscriber role each week (eg "lesson #1 subscriber" becomes "lesson #2 subscriber", etc).
• Use a newsletter module to email subscribers with links to their latest lesson.

It works and I'm impressed with Ubercart and the other modules, but I keep thinking there must be a better way. I've looked at a lot of Drupal modules and forum topics, and even checked out other CMSes, LMSes and shopping carts, but I can't bring myself to leave Drupal and Ubercart.
Does anyone know of a better solution? Although the Administer Users by Role module makes changing roles much easier, something automated would be ideal.
In the long term, adding these features (with automation) to Ubercart/Workflow.ng could be appealing to a lot of people in the online training area. However, in the meantime, I'm open to any ideas, big or small.
Thanks.