Using Live Customizer and Compiling SCSS File by Ajax

I’ll second that as an awesome idea. Haven’t ever seen it done…

I’ve just done something similar. Hook into the ‘wp_scss_variables’ filter, fetch your theme mods or options, and return them. Then hook into the ‘wp_scss_needs_compiling’ filter and decide whether or not SASS needs recompiling. I’ve used update_option(‘wp_scss_needs_compiling’, true) on ‘customize_preview_init’ and ‘customize_save_after’.