From what I can tell it looks like that the sitemap is using some javascript that is hard coded to use the cID. I'd agree that it would be nice for this to be implemented. I doubt you have much luck with getting a pull request into the 5.6 version, but you could always suggest it be implemented in 5.7 (if it isn't already?)
Here's the code where it's getting set currently in 5.6: https://github.com/concrete5/concrete5/blob/51badcdb1f00b5ba07ba11e02e4422e76da107ac/web/concrete/js/ccm_app/sitemap.js#L45
And here's where you can report an issue for the new 5.7 version if you'd like to see it implemented: https://github.com/concrete5/concrete5-5.7.0/issues