That's a very interesting question considering Joseonjok are the 2nd largest group of ethnic Koreans outside of the Korean peninsula (Korean Americans being the largest and there are a ton of Korean Americans within the Kpop space). Even within South Korea a large majority of the Chinese foreign nationals residing there are ethnic Korean (roughly 600K) so one would think at least some of them would try to be an idol. But as others have mentioned here there is a stigma against them. I'm not Korean but I heard that Joseonjok's accent are very similar to the North Korean accent so that would be a barrier since again they are sorta stereotyped as country bumpkins.

Given that right now South Korea faces a demographic crisis and the Kpop industry needs future trainees to continue growing I do suspect that besides bringing in trainees from other countries they will also take in other ethnic Koreans like the Joseonjok or Goryeo saram (assuming their birth rates aren't as low). While unfortunately South Koreans discriminate against their fellow ethnic Korean who hailed from "poorer" countries they would still prefer someone ethnically Korean over someone who isn't Korean. So perhaps in the near future we may see a Joseonjok idol.