I have not seen a mechanical cruise control device for a thumb throttle but that doesn't mean they don't exist. In my case, I swapped my thumb type for a half twist throttle. It was a cheap & easy project. I don't know if this is possible on your bike however. Once the twist throttle was installed, I bought this mechanical cruise control device which works perfectly:
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Keep in mind, this is a mechanical device and isn't true cruise control since it doesn't compensate for changes in slope, wind or surface conditions. I use it mainly while on long 50+ mile rides to maintain speed while I take a rest from pedaling or to get some butt relief while standing on the pedals. This avoids time lost during rest stops.
I also use the throttle when trying to sync speed with other riders. I set the throttle to match and then apply whatever pedal effort I choose. This way, it's used as an infinitely adjustable PAS with me assisting the bike rather than it assisting me.
All this being said, I don't use the throttle that often. As you say, it's handy to have under certain conditions. It's also a form of insurance for me since I can always get back if I have a physical issue which keeps me from being able to pedal.
Love 'em or hate 'em, a throttle is simply a tool that, in my case, I wouldn't be without.