Theme override function not working

I don't know if this is a theme issue or a mobile_tools issue, but since mobile_tools is involved I'm starting here.

I have mobile_tools set up and a mobile specific theme set up. All it's used for one particular product in Ubercart. We have an iPad/iPhone app that opens up a checkout page for one product in a browser window so the user can complete checkout and then the user is redirected back into the app proper once the sale is completed. The pages show up as intended in the browser window, so mobile_tools is working perfectly for that aspect (which is awesome!).

The problem is that I want to re-theme the checkout completed page, which should be easy as pie because there's a theme_uc_cart_checkout_complete function that I should be able to override in my template.php file with a MYTHEME_uc_cart_checkout_complete function, which I have done.

Only that function is never executed and the checkout content is never converted. Perhaps my understanding of overriding functions in template.php is wrong. It's my understanding that putting in the function in the template.php with the above naming convention (replacing the 'theme' with my theme name) is all that is necessary to get the function to override. Is that the case or is there something else I am missing? (Just to be thorough, I am clearing the caches in between trial runs.)

If I do have that all correct, is this an issue with mobile_tools not fully using the theming engine properly?