Best OOP method for a layout library
The core of your question is whether or not you should use setters ($tpl->setSomeParam()) or a simple array to bind template parameters. While using an array my not seem very OO, it's actually a pretty effective way of decoupling your data from your templates. As you've discovered, having to add a new setter to your layout manager every time you add a new parameter to one of your templates means the layout manager is tightly coupled to all of your templates – not good.

This is pretty common practice these days. Twig, Underscore.js, and many others do the same thing.