So, back to your dilemma. What about a
lockable luggage strap? I know it's not quite as theft-proof as a bike cable, but... if you wrapped it up with the lock end in the center, it and threw a rubber band around it so it doesn't come unraveled in your backpack until you are ready to use it in the Parks... It's a just a thought - I know it isn't as perfect a solution as the bike cable/lock, but since that first choice won't make it through bag check, something like this might. (I just searched for "locking strap" on
Amazon, and found that, btw)
There is one last option I thought of - an accessory that you could perhaps install somewhere: a
locking storage box like you will sometimes see on motorcycles (and some personal scooters). I don't know if that's even a reasonable option - but I included the search results so you could see different shapes and sizes.
Good luck with finding a way to (safely & securely) keep your stuff secured on your
scooter. And let us know what you come up with; I'm sure that there are lots of folks here who might want to take a page from your book!