Re: concerts and libraries, you meet people outside of them (for concerts, in line for them, or in the pit waiting for the show to start), not at them per se.
I don't know about "hacking camps" (hackathons? code boot camps?) — never been to one — but a close cousin of these, [industry niche] startup events, tend to attract just as many business cofounders as technical cofounders; and the business cofounders tend to be an even gender split. (They also attract seed-stage VC scouts and marketing people, and these jobs actually tend to lean female.)
> As I said, I do, but none of the things I like involve enough people of the opposite sex.
So... take up some new hobbies?
And when I say that, I don't mean to work "bottom up" starting from what you might like; but rather, "top down", starting from what hobbies your ideal partner might have, and then filtering those for which ones you feel at least amicably neutral about. E.g. if you feel like you'd enjoy being with someone who's also 50yo and "creative", and you're not against e.g. painting, or pottery, or creative writing; then attend a continuing-education course on one of those. Because that's where you'll find other 50yos trying to learn those things.