Content from another site

plugin.php

<?php
define( 'PRODUCT_QUERY_VAR', 'product__dbernar1' );
define( 'PRODUCT_URL_PREFIX', 'products' );

function get_permalink__dbernar1( $post = null ) {
        if ( is_null( $post ) ) global $post;
        return site_url( PRODUCT_URL_PREFIX . '/' . $post->post_name );
}

add_action( 'init', 'create_post_type_rewrite_rules__dbernar1' );

function create_post_type_rewrite_rules__dbernar1() {
        add_rewrite_tag( '%' . PRODUCT_QUERY_VAR . '%', '([^&]+)' );
        add_rewrite_rule( '^' . PRODUCT_URL_PREFIX .' /([^/]*)/?', 'index.php?' . PRODUCT_QUERY_VAR . '=$matches[1]', 'top' );
}

add_action( 'wp', 'show_product_in_current_site__dbernar1' );

function show_product_in_current_site__dbernar1() {
        $viewing_product = get_query_var( PRODUCT_QUERY_VAR );

        if ( $viewing_product ) {
                allow_theme_to_overwrite_template__dbernar1( $named = 'single-product.php' );
        }
}

function allow_theme_to_overwrite_template__dbernar1( $template_filename ) {
        if ( $overridden_template = locate_template( $template_filename ) ) {
                load_template( $overridden_template );
        } else {
                load_template( dirname( __FILE__ ) . '/templates/' . $template_filename );
        }
        die;
}

This is missing other pieces to actually be a whole solution, I created it to fill in the blanks of a guy who was already partway there.

Get in touch ( in a way listed at http://gravatar.com/dbernar1 ) if you need more info.