AWESOME THREAD!
The SAK/Victorinox solution is the best IMO because of the unmistakable red scales and cross-in-the-shield logo. Everyone knows it as well as they do the McDonald's arches. IOW, iconic. But most of all, innocuous. And we're still in a generation whose grandfathers, at least, carried a little knife of some sort, and it likely looked something like you're wanting.
So if you go with SAK, and you should, go with the red scales. Also, go a bit larger than a Classic as you'll want the extra tools like bottle opener and awl and maybe even saw. Scissors are a great way to go for so many tasks. Urbanites otherwise put off by knives will be comfortable with them. And nothing screams, "Not a weapon!" like scissors on a pocketknife.
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Oh, and a lock is OK, as people keep saying, but most of your "sheeple" will try, fumble, slip, drop it, or otherwise fail at closing the blade when handing it back to you. If you have to step away while they're using it and they finish their task while you're away, you are likely to come back to a knife sitting open or stuck blade first into some object because they couldn't understand the mechanism. You're right to go slipjoint.
Zieg