Accessing post data before functions.php in Wordpress

I am updating my custom Wordpress theme, but as it is rather time consuming process I would like to launch one section at a time. In other words there would be several versions of a theme for different parts of the website. To keep things tidy I would like to keep them in separate folders, with all the assets such as js, images, css.

I managed to rewrite template hierarchy using conditional tags but got stuck on functions.php

I was trying to use custom filed (post meta) to switch between several functions.php files but unfortunately $post is not available there so I am unable to use get_post_meta().

I could only find a trace of a solution with custom db queries, $wpdb etc. but can't really figure it out.

Is there any fairly simple solution to hook-up into post data (wp_query) before functions.php is loaded? Or to somehow differently modify where the functions are loaded from?

To illustrate what I'm writing about I pasted my main index.php

<?
get_header();

/*
 * Get theme version according to the custom field 'section'
 */
 
if( function_exists ( 'theme_version' ) ){
        $theme = theme_version( @get_the_ID() );
} else {
        $theme = 'v2';
}

include_once( 'theme/'. $theme .'/TEMPLATE_BUILDER.php' );


get_footer();

?>

Thanks!