Getting template_include to work when allow_url_include is off?

Wouldn’t ya know – 10 minutes or less since I asked my question and I’ve discovered the answer.

I was attempting to define MY_PLUGIN_TEMPLATES using using plugins_dir_url(), now it looks like this and is working:

define( 'MY_PLUGIN_TEMPLATES', dirname( __FILE__ ) . '/templates' );

Thanks to mfields for the suggestion of dirname instead of WP_PLUGIN_DIR.