How can I use Smarty templates with A/B testing?

I am attempting to build a little modification in our code to allow easier A/B testing.

I'd like to know if I can somehow 1. have my regular code under the /templates directory 2. have any a/b code under /templates/_abtests/, but also follow the same hierarchy as the regular code. for example... an ab test can overwrite a file like '/templates/foo.tpl', and use instead '/templates/_abtests/testfoo/foo.tpl'

I tried changing the template directory when in a test. Right before calling the display method, I would check if a user is in a test, and if so, set up the template_dir accordingly. I'd assign an array with the 'ab' directory first, then the default. I am using Smarty2.

the problem with this is that it caches the first instance, and uses that as the template for the baseline and ab test case. ie: i have a parameter to force me into a test bucket, but the template is the same.

thoughts on how to achieve this? goal is to not have to add a bunch of template hooks (if/else) in the templates. and achieve this by simple template/file includes.