There were people once upon a time—and maybe even now—who invested in North Korean debt. I have not done that, but it may be another way that people can invest in North Korea. I don't even know if North Korean debt still trades, but it was defaulted on at some point.
Stamps and coins were the only ways I knew of that one could get some sort of exposure. This is because you are not investing in the country, obviously, because you are buying them in a secondary or tertiary market. That said, I think the US government is going to make owning stamps illegal too.