Having been a Locksmith for 18 yrs, I've made many lockout tools. Good job on this one. Most lockout tools are very stiff so a ordinary metal coat hanger usually will not due. I've used 1/8" non-flux coated welding rod to make many of my tools. They make your hands dirty but they work well and only break if your bend them too tight.
An auxillary release is a great idea and very simple. You can get a replacement lawn mower throttle cable and braket for under $10. AS USUAL, another mod on my "to-do" list.
, I invented a tool to open the Mercedes S class vehicles without damage in 1994. Up to that time, locksmiths around the world had damaged about 12 cars totalling dealer repairs of over $30,000.00 to their insurance claims
. Dumbass me didn't patent the tool!